Check the session format validity in the gRPC request handler using the simple validate function, as we have for deviceID in D2978
Stacked on top of [[ https://phabricator.ashoat.com/D3124 | D3124 ]].
Related linear task [[ https://linear.app/comm/issue/ENG-650/sessionid-validation-in-tunnelbroker | ENG-650 ]]