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/paths/src/paths.rs

@@ -212,6 +212,20 @@ pub fn copilot_dir() -> &'static PathBuf {
     COPILOT_DIR.get_or_init(|| support_dir().join("copilot"))
 }
 
+pub fn copilot_chat_config_path() -> &'static PathBuf {
+    static COPILOT_CHAT_CONFIG_DIR: OnceLock<PathBuf> = OnceLock::new();
+
+    COPILOT_CHAT_CONFIG_DIR.get_or_init(|| {
+        if cfg!(target_os = "windows") {
+            home_dir().join("AppData")
+        } else {
+            home_dir().join(".config")
+        }
+        .join("github-copilot")
+        .join("hosts.json")
+    })
+}
+
 /// Returns the path to the Supermaven directory.
 pub fn supermaven_dir() -> &'static PathBuf {
     static SUPERMAVEN_DIR: OnceLock<PathBuf> = OnceLock::new();