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/copilot/Cargo.toml

@@ -13,6 +13,8 @@ path = "src/copilot.rs"
 doctest = false
 
 [features]
+default = []
+schemars = ["dep:schemars"]
 test-support = [
     "collections/test-support",
     "gpui/test-support",
@@ -26,13 +28,16 @@ test-support = [
 anyhow.workspace = true
 async-compression.workspace = true
 async-tar.workspace = true
+chrono.workspace = true
 collections.workspace = true
 client.workspace = true
 command_palette_hooks.workspace = true
 editor.workspace = true
+fs.workspace = true
 futures.workspace = true
 gpui.workspace = true
 http_client.workspace = true
+isahc.workspace = true
 language.workspace = true
 lsp.workspace = true
 menu.workspace = true
@@ -41,8 +46,12 @@ parking_lot.workspace = true
 paths.workspace = true
 project.workspace = true
 serde.workspace = true
+serde_json.workspace = true
+schemars = { workspace = true, optional = true }
+strum.workspace = true
 settings.workspace = true
 smol.workspace = true
+task.workspace = true
 ui.workspace = true
 util.workspace = true
 workspace.workspace = true