This diff introduces handling by the gRPC server of the `NewNotifyToken` message [[ https://github.com/CommE2E/comm/blob/50d12886e7e757fdb876a84ddcda9b98382bf460/shared/protos/tunnelbroker.proto#L112 | message ]] from the client in the `MessagesStream` bidirectional stream.
When the client sends the new notification token the server should save it for the client in the database sessions table.
Linear task: [[ https://linear.app/comm/issue/ENG-2060/implement-tunnelbroker-grpc-messagesstream-api-handler-in-rust | ENG-2060 ]]