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/zed/src/zed.rs

@@ -3456,6 +3456,11 @@ mod tests {
             project_panel::init((), cx);
             outline_panel::init((), cx);
             terminal_view::init(cx);
+            copilot::copilot_chat::init(
+                app_state.fs.clone(),
+                app_state.client.http_client().clone(),
+                cx,
+            );
             language_model::init(app_state.client.clone(), cx);
             assistant::init(app_state.fs.clone(), app_state.client.clone(), cx);
             repl::init(