(feat) Add support for OpenAi's new "responses" API

[danwritecode]

Currently llmkit only supports chat completion endpoints. I would like to extend support to the new responses API, as least so far as llmkit is concerned (we may not care about image generation for example).

[gooselord-0]

I’ll take this if it’s still open

[danwritecode]

It's all yours my friend. For now let's only focus on the API side of the house. We'll have the UI continue to use the chat completion endpoints.

If you have any questions let me know. The chat completion endpoints (controllers/prompts.rs/api_completions) should serve as a pretty good starting point.