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/language_models/Cargo.toml

@@ -28,6 +28,7 @@ credentials_provider.workspace = true
 copilot.workspace = true
 deepseek = { workspace = true, features = ["schemars"] }
 editor.workspace = true
+feature_flags.workspace = true
 fs.workspace = true
 futures.workspace = true
 google_ai = { workspace = true, features = ["schemars"] }