Add feature flag to use cloud.zed.dev instead of llm.zed.dev (#34076)

This PR adds a new `zed-cloud` feature flag that can be used to send
traffic to `cloud.zed.dev` instead of `llm.zed.dev`.

This is just so Zed staff can test the new infrastructure. When we're
ready for prime-time we'll reroute traffic on the server.

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -92,6 +92,17 @@ impl FeatureFlag for JjUiFeatureFlag {
     const NAME: &'static str = "jj-ui";
 }
 
+pub struct ZedCloudFeatureFlag {}
+
+impl FeatureFlag for ZedCloudFeatureFlag {
+    const NAME: &'static str = "zed-cloud";
+
+    fn enabled_for_staff() -> bool {
+        // Require individual opt-in, for now.
+        false
+    }
+}
+
 pub trait FeatureFlagViewExt<V: 'static> {
     fn observe_flag<T: FeatureFlag, F>(&mut self, window: &Window, callback: F) -> Subscription
     where