Define an OpenAPI.

# Macro parameters

These are attributes that can be added to the `#[OpenApi]` attribute.

| Attribute       | Description                                                                                                      | Type                                                       | Optional |
|-----------------|------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------|----------|
| prefix_path     | Define the prefix for all operation paths. May contain shared path parameters.                                   | string                                                     | Y        |
| tag             | Define a tag for all operations. This must be the name of an in-scope variant of an enum which implements `Tags` | Tags                                                       | Y        |
| response_header | Add an extra response header to all operations.                                                                  | [`ExtraHeader`](macro@ApiResponse#extra-header-parameters) | Y        |
| request_header  | Add an extra request header to all operations.                                                                   | [`ExtraHeader`](macro@ApiResponse#extra-header-parameters) | Y        |

## Example

```rust
use poem_openapi::{OpenApi, Tags};
use poem_openapi::param::Path;
use poem_openapi::payload::PlainText;

#[derive(Tags)]
enum MyTags {
    V1
}

struct Api;

#[OpenApi(prefix_path = "/v1/:customer_id", tag = "MyTags::V1")]
impl Api {
    /// Greet the customer
    ///
    /// # Example
    /// 
    /// Call `/v1/1234/hello` to get the response `"Hello 1234!"`. 
    #[oai(path = "/hello", method = "get")]
    async fn hello(&self, customer_id: Path<String>) -> PlainText<String> {
        PlainText(format!("Hello {}!", customer_id.0))
    }
}
```

# Operation parameters

Parameters that can be passed into the `#[oai()]` attribute above each operation function within an `OpenApi`.

| Attribute       | Description                                                                                                          | Type                                                       | Optional |
|-----------------|----------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------|----------|
| path            | URI path optionally containing path parameters (e.g., "/:name/hello")                                                | string                                                     | N        |
| method          | HTTP method. The possible values are "get", "post", "put", "delete", "head", "options", "connect", "patch", "trace". | string                                                     | N        |
| deprecated      | Operation deprecated                                                                                                 | bool                                                       | Y        |
| external_docs   | Specify a external resource for extended documentation                                                               | string                                                     | Y        |
| tag             | Tag to use for an operation. Must be a variant of an enum which implements `Tags`                                    | Tags                                                       | Y        |
| operation_id    | Unique string used to identify the operation.                                                                        | string                                                     | Y        |
| transform       | Use a function to transform the API endpoint.                                                                        | string                                                     | Y        |
| response_header | Add an extra response header to the operation.                                                                       | [`ExtraHeader`](macro@ApiResponse#extra-header-parameters) | Y        |
| request_header  | Add an extra request header to all operations.                                                                       | [`ExtraHeader`](macro@ApiResponse#extra-header-parameters) | Y        |
| actual_type     | Specifies the actual response type                                                                                   | string                                                     | Y        |
| code_samples    | Code samples for the operation                                                                                       | object                                                     | Y        |
| hidden          | Hide this operation in the document                                                                                  | bool                                                       | Y        |

## Example

```rust
use poem_openapi::{OpenApi, Tags};
use poem_openapi::param::Path;
use poem_openapi::payload::PlainText;

#[derive(Tags)]
enum MyTags {
    V1
}

struct Api;

#[OpenApi]
impl Api {
    /// Greet the customer
    ///
    /// # Example
    /// 
    /// Call `/v1/1234/hello` to get the response `"Hello 1234!"`. 
    #[oai(path = "/v1/:customer_id/hello", method = "get", tag = "MyTags::V1")]
    async fn hello(&self, customer_id: Path<String>) -> PlainText<String> {
        PlainText(format!("Hello {}!", customer_id.0))
    }
}
```

# Operation argument parameters

| Attribute                | Description                                                                                                                                                                                                                                           | Type                                      | Optional          |
|--------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------|-------------------|
| name                     | Parameter name                                                                                                                                                                                                                                        | string                                    | Y                 |
| deprecated               | Argument deprecated                                                                                                                                                                                                                                   | bool                                      | Y                 |
| default                  | Default value                                                                                                                                                                                                                                         | bool,string                               | Y                 |
| explode                  | When this is `true`, parameter values of type array or object generate separate parameters for each value of the array or key-value pair of the map.                                                                                                  | bool                                      | Y (default: true) |
| validator.multiple_of    | The value of "multiple_of" MUST be a number, strictly greater than 0. A numeric instance is only valid if division by this value results in an integer.                                                                                               | number                                    | Y                 |
| validator.maximum        | The value of "maximum" MUST be a number, representing an upper limit for a numeric instance. If `exclusive` is `true` and instance is less than the provided value, or else if the instance is less than or exactly equal to the provided value.      | { value: `<number>`, exclusive: `<bool>`} | Y                 |
| validator.minimum        | The value of "minimum" MUST be a number, representing a lower limit for a numeric instance. If `exclusive` is `true` and instance is greater than the provided value, or else if the instance is greater than or exactly equal to the provided value. | { value: `<number>`, exclusive: `<bool>`} | Y                 |
| validator.max_length     | The value of "max_length" MUST be a non-negative integer. A string instance is valid against this validator if its length is less than, or equal to, the value.                                                                                       | usize                                     | Y                 |
| validator.min_length     | The value of "min_length" MUST be a non-negative integer.  The value of this validator MUST be an integer. This integer MUST be greater than, or equal to, 0.                                                                                         | usize                                     | Y                 |
| validator.pattern        | The value of "pattern" MUST be a string. This string SHOULD be a valid regular expression, according to the ECMA 262 regular expression dialect. A string instance is considered valid if the regular expression matches the instance successfully.   | string                                    | Y                 |
| validator.max_items      | The value of "max_items" MUST be an integer. This integer MUST be greater than, or equal to, 0. An array instance is valid if its size is less than, or equal to, the value of this validator.                                                        | usize                                     | Y                 |
| validator.min_items      | The value of "min_items" MUST be an integer. This integer MUST be greater than, or equal to, 0. An array instance is valid if its size is greater than, or equal to, the value of this validator.                                                     | usize                                     | Y                 |
| validator.unique_items   | The value of "unique_items" MUST be an boolean.  If this value is `false`, the instance validates successfully.  If this value is `true`, the instance validates successfully if all of its elements are unique.                                      | bool                                      | Y                 |
| validator.max_properties | The value of this keyword MUST be a non-negative integer. An object instance is valid against "maxProperties" if its number of properties is less than, or equal to, the value of this keyword.                                                       | usize                                     | Y                 |
| validator.min_properties | The value of this keyword MUST be a non-negative integer. An object instance is valid against "minProperties" if its number of properties is greater than, or equal to, the value of this keyword.                                                    | usize                                     | Y                 |

# Examples

```rust
use poem_openapi::{
    param::Header,
    payload::{Json, PlainText},
    ApiRequest, Object, OpenApi, ApiResponse,
};

#[derive(Object)]
struct Pet {
    id: String,
    name: String,
}

#[derive(ApiRequest)]
enum CreatePetRequest {
    /// This request receives a pet in JSON format(application/json).
    CreateByJSON(Json<Pet>),
    /// This request receives a pet in text format(text/plain).
    CreateByPlainText(PlainText<String>),
}

#[derive(ApiResponse)]
enum CreatePetResponse {
    /// Returns when the pet is successfully created.
    #[oai(status = 200)]
    Ok,
    /// Returns when the pet already exists.
    #[oai(status = 409)]
    PetAlreadyExists,
}

struct PetApi;

#[OpenApi]
impl PetApi {
    /// Create a new pet.
    #[oai(path = "/pet", method = "post")]
    async fn create_pet(
        &self,
        #[oai(name = "TOKEN")] token: Header<String>,
        req: CreatePetRequest
    ) -> CreatePetResponse {
        todo!()
    }
}
```