Add OpenAI as a pluggable spec

[lantiga]

🚀 Feature

We should enable serving a model through a spec, without having to implement it manually in decode_request and encode_response. The spec (could be more than one) would:

• expose a route
• implement specific ways of decoding requests and encoding responses
• require the API to expose certain kinds of information (e.g. token used)

in a way that is pluggable at the LitServer level (spec=OpenAISpec) and independent from the API implementation itself.

Motivation

We want to make it seamless for users to expose a model using one or more standard specs.

Pitch

I define a LitAPI subclass, call LitServer(api, spec=OpenAISpec, ...) and I will get an v1/chat/completions/ endpoint that behaves like an OpenAI compatible endpoint.

Alternatives

We subclass LitServer and LitAPI, but this would't compose cleanly down the road with other pieces we want to factor out (e.g. kvcache management).

[lantiga]

Closed by #98 #100 #101