Simplify Model/Provider onboarding

[krrishdholakia]

Starting a discussion to track ways we can make it easier for the community to contribute new providers/models.

Ideally this would make it easy for users to contribute translation logic, and abstract the http logic away.

The new rerank onboarding is our initial attempt at this - https://docs.litellm.ai/docs/adding_provider/new_rerank_provider