Simplify LLM protocol (#15366)

In this pull request, we change the zed.dev protocol so that we pass the
raw JSON for the specified provider directly to our server. This avoids
the need to define a protobuf message that's a superset of all these
formats.

@bennetbo: We also changed the settings for available_models under
zed.dev to be a flat format, because the nesting seemed too confusing.
Can you help us upgrade the local provider configuration to be
consistent with this? We do whatever we need to do when parsing the
settings to make this simple for users, even if it's a bit more complex
on our end. We want to use versioning to avoid breaking existing users,
but need to keep making progress.

```json
"zed.dev": {
  "available_models": [
    {
      "provider": "anthropic",
        "name": "some-newly-released-model-we-havent-added",
        "max_tokens": 200000
      }
  ]
}
```

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/collab/src/lib.rs

@@ -1,4 +1,3 @@
-pub mod ai;
 pub mod api;
 pub mod auth;
 pub mod db;