Server-sent feature flags (#2894)

This PR adds the wiring to both the server and the client for sending
and receiving individual feature flags, as well as a client side API for
convenient access to these feature flags.

Release Notes:

- N/A

crates/rpc/proto/zed.proto

@@ -1114,6 +1114,7 @@ message GetPrivateUserInfo {}
 message GetPrivateUserInfoResponse {
     string metrics_id = 1;
     bool staff = 2;
+    repeated string flags = 3;
 }
 
 // Entities