Show current plan in user menu (#15513)

This PR updates the user menu to show the user's current plan.

Also adds a new RPC message to send this information down to the client
when Zed starts.

This is behind a feature flag.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

Cargo.lock

@@ -11186,6 +11186,7 @@ dependencies = [
  "dev_server_projects",
  "editor",
  "extensions_ui",
+ "feature_flags",
  "feedback",
  "gpui",
  "http_client",