Create shared openai-compatible inference adapter

[russellb]

There are multiple open PRs looking to add inference adapters for services that offer OpenAI-compatible APIs.

• databricks - add databricks provider #83
• sambanova - Add sambanova provider #136
• runpod - Add Runpod Provider #157
• vllm - WIP: Add vLLM provider #178

These adapters all use the OpenAI client and have the same config options (a URL and API key).

Options for moving forward:

1. Create a single shared adapter for OpenAI-compatible API endpoints.
2. If some cases require additional unique configuration, most of the code could be shared.

[ashwinb]

I support creating a shared code but not a shared adapter. I think implementation quality in terms of faithfulness of precise token-level behavior (on the backend) varies so it is best if we keep a top-level fork but share underlying code where it makes sense. I also plan to put together a set of tests which test tool calling behavior and other corner-case behavior we know can be problematic.

Please do hold off on doing this work for a couple days. There's some impending refactors incoming. I will be merging some PRs in the meantime to avoid duplicate work on behalf of the providers.

[terrytangyuan]

@ashwinb I have put together some work on this before noticing this issue and your comment: #195

Is it possible to merge that effort and then move them to reusable code in a separate PR (or as part of your incoming impending refactors) if a shared adapter is not preferred? It's a working adapter. This will unblock some of our existing tasks and avoid duplicating work.