Add GitHub token environment variable support for Copilot (#31392)

Add support for environment variables as authentication alternatives to
OAuth flow for Copilot. Closes #31172

We can include the token in HTTPS request headers to hopefully resolve
the rate limiting issue in #9483. This change will be part of a separate
PR.

Release Notes:

- Added support for manually providing an OAuth token for GitHub Copilot
Chat by assigning the GH_COPILOT_TOKEN environment variable

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/copilot/src/copilot_chat.rs

@@ -16,6 +16,8 @@ use paths::home_dir;
 use serde::{Deserialize, Serialize};
 use settings::watch_config_dir;
 
+pub const COPILOT_OAUTH_ENV_VAR: &str = "GH_COPILOT_TOKEN";
+
 #[derive(Default, Clone, Debug, PartialEq)]
 pub struct CopilotChatSettings {
     pub api_url: Arc<str>,
@@ -405,13 +407,19 @@ impl CopilotChat {
         })
         .detach_and_log_err(cx);
 
-        Self {
-            oauth_token: None,
+        let this = Self {
+            oauth_token: std::env::var(COPILOT_OAUTH_ENV_VAR).ok(),
             api_token: None,
             models: None,
             settings,
             client,
+        };
+        if this.oauth_token.is_some() {
+            cx.spawn(async move |this, mut cx| Self::update_models(&this, &mut cx).await)
+                .detach_and_log_err(cx);
         }
+
+        this
     }
 
     async fn update_models(this: &WeakEntity<Self>, cx: &mut AsyncApp) -> Result<()> {