This diff introduces an optional field to get the new notification token from the client.
In case of the client has an expired or wrong notification token we will send an gRPC (9) `FAILED_PRECONDITION` error to the client and will expect that he will re-request the `Get` request with the `newNotifyToken`.
Related Linear task: [[ https://linear.app/comm/issue/ENG-1816/update-tunnelbroker-api-to-support-sending-notices-to-clients-for | ENG-1816 ]]