Part of ENG-8137.
Addresses ENG-7215 on native.
Modified native_rust_library and native Identity client to use username received in AuthResponse message from login/registration RPCs.
WARNING: We should first deploy Identity with D12127 before landing this diff. Otherwise, the Identity won't be sending username in responses and gRPC will default this to empty string, which potentially breaks all login / registration actions.
Depends on D12127