language_models: Dynamically detect Copilot Chat models (#29027)

I noticed the discussion in #28881, and had thought of exactly the same
a few days prior.

This implementation should preserve existing functionality fairly well.

I've added a dependency (serde_with) to allow the deserializer to skip
models which cannot be deserialized, which could occur if a future
provider, for instance, is added. Without this modification, such a
change could break all models. If extra dependencies aren't desired, a
manual implementation could be used instead.

- Closes #29369 

Release Notes:

- Dynamically detect available Copilot Chat models, including all models
with tool support

---------

Co-authored-by: AidanV <aidanvanduyne@gmail.com>
Co-authored-by: imumesh18 <umesh4257@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/language_models/Cargo.toml

@@ -15,13 +15,15 @@ path = "src/language_models.rs"
 anthropic = { workspace = true, features = ["schemars"] }
 anyhow.workspace = true
 aws-config = { workspace = true, features = ["behavior-version-latest"] }
-aws-credential-types = { workspace = true, features = ["hardcoded-credentials"] }
+aws-credential-types = { workspace = true, features = [
+    "hardcoded-credentials",
+] }
 aws_http_client.workspace = true
 bedrock.workspace = true
 client.workspace = true
 collections.workspace = true
 credentials_provider.workspace = true
-copilot = { workspace = true, features = ["schemars"] }
+copilot.workspace = true
 deepseek = { workspace = true, features = ["schemars"] }
 editor.workspace = true
 feature_flags.workspace = true