Add support for structured LLM output

[adambalogh]

https://developers.openai.com/api/docs/guides/structured-outputs

[Ridwannurudeen]

Hi! I'd like to work on this. I'll add \ support (matching the OpenAI structured outputs spec) to the \ and \ methods, threading it through the internal async methods and adding CLI support + examples. Happy to adjust the approach based on feedback.

[Ridwannurudeen]

Hi! I would like to work on this. I will add response_format support (matching the OpenAI structured outputs spec) to the chat() and completion() methods, threading it through the internal async methods and adding CLI support + examples. Happy to adjust the approach based on feedback.