Add support for Vercel as a language model provider (#33292)

Vercel v0 is an OpenAI-compatible model, so this is mostly a dupe of the
OpenAI provider files with some adaptations for v0, including going
ahead and using the custom endpoint for the API URL field.

Release Notes:

- Added support for Vercel as a language model provider.

crates/language_models/src/provider.rs

@@ -9,3 +9,4 @@ pub mod mistral;
 pub mod ollama;
 pub mod open_ai;
 pub mod open_router;
+pub mod vercel;