OpenAI API compatibility for all providers

[kisamoto]

Currently this API purely acts a proxy so anybody willing to integrate with different providers needs to also implement an SDK for each provider in their code (my use case).

A huge appealing factor would be if we could use only the OpenAI SDK and purely by changing the URLs when initializing a new client get access to the different providers. Side note, it would also play nicely with #40 as the OpenAI client would handle those errors.

Perhaps each provider could be under a separate compatability URL e.g. http://localhost:8002/api/compat/openai/providers/{{ provider_slug }}/

Ollama implemented something similar recently via a middleware which maps OpenAI API objects to their own internal representation if you need some inspiration.