Support for All OpenAI Compatible API Endpoints (RFC)

Nice work on this! Something that's lacking for me is the ability to use third-party API providers that use OpenAI-compatible API formats, such as DeepInfra. I would be happy to PR this if it'd be welcome -- and can build out a proposal for how I'd implement as well. At a high level, it would function by allowing any model name when `openai_api_url` and `openai_api_key` are provided. It would also include updates to the documentation to note this new behavior.

I'm not an expert here on OpenAI compatible APIs but they seem very popular from my research -- DeepInfra, which is what I'm using and the most familiar with, does have support for Vercel's `ai` SDK, including support for tools (like search). https://deepinfra.com/docs/advanced/aisdk

Is there anything else I'm missing that should be considered to add support for this? I took ~15 min to run through the code and don't see any glaring issues, but lmk if I'm wrong. Appreciate you!