Add GitHub Copilot Chat Support (#14842)

# Summary

This commit implements Github Copilot Chat support within the existing
Assistant panel/framework. It required a little bit of trickery and
internal API modification, as Copilot doesn't use the same
authentication-style as all of the existing providers, opting to use
OAuth and a short lived API key instead of a straight API key. All
existing Assistant features should work.

Release Notes:
- Added Github Copilot Chat support
([#4673](https://github.com/zed-industries/zed/issues/4673)).

## Screenshots
<img width="1552" alt="A screenshot showing a conversation between a
user and Github Copilot Chat within the Zed editor."
src="https://github.com/user-attachments/assets/73eaf6a2-792b-4c40-a7fe-f763bd6417d7">

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>

crates/assistant/src/assistant_settings.rs

@@ -380,6 +380,7 @@ fn providers_schema(_: &mut schemars::gen::SchemaGenerator) -> schemars::schema:
             "ollama".into(),
             "openai".into(),
             "zed.dev".into(),
+            "copilot_chat".into(),
         ]),
         ..Default::default()
     }
@@ -419,7 +420,7 @@ pub struct AssistantSettingsContentV1 {
     default_height: Option<f32>,
     /// The provider of the assistant service.
     ///
-    /// This can either be the internal `zed.dev` service or an external `openai` service,
+    /// This can be "openai", "anthropic", "ollama", "zed.dev"
     /// each with their respective default models and configurations.
     provider: Option<AssistantProviderContentV1>,
 }