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/copilot/Cargo.toml

@@ -14,7 +14,6 @@ doctest = false
 
 [features]
 default = []
-schemars = ["dep:schemars"]
 test-support = [
     "collections/test-support",
     "gpui/test-support",
@@ -43,16 +42,15 @@ node_runtime.workspace = true
 parking_lot.workspace = true
 paths.workspace = true
 project.workspace = true
-schemars = { workspace = true, optional = true }
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true
-strum.workspace = true
 task.workspace = true
 ui.workspace = true
 util.workspace = true
 workspace.workspace = true
 workspace-hack.workspace = true
+itertools.workspace = true
 
 [target.'cfg(windows)'.dependencies]
 async-std = { version = "1.12.0", features = ["unstable"] }