The initial system to send to ChatGPT API. This can be used to set the initial context of the conversation, and
instruct ChatGPT to use a specific persona.

* Property: `systemMessage`
* Type: `string`
* Required: `false`
* Usage:

```js
const adapter = useUnsafeChatAdapter({
    systemMessage: 'I want you to act as a customer support agent.'
});
```
