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>

crates/proto/src/proto.rs

@@ -359,6 +359,7 @@ messages!(
     (UpdateParticipantLocation, Foreground),
     (UpdateProject, Foreground),
     (UpdateProjectCollaborator, Foreground),
+    (UpdateUserPlan, Foreground),
     (UpdateWorktree, Foreground),
     (UpdateWorktreeSettings, Foreground),
     (UsersResponse, Foreground),