Add support for response_format

[adinapoli]

Morning @agrafix 😉

Recent versions of OpenAI added JSON mode.

Passing a response_format object in a chat completion request ensures that OpenAI will try to respond with valid JSON.

I have extended a ChatCompletionRequest with this field which is optional anyway (as per documentation), so these changes are all backward compatibile.

Thanks!

[nickhs]

This is great! One small concern is that there's lots of response_format across different APIs. Maybe rename to ChatResponseFormat?

[nickhs]

merged - I'll do a quick rename myself. Thanks @adinapoli!