AUTH command not supported (IDFGH-11745)

[papadeltasierra]

MQTT v5 has an AUTH command that allows reauthentication without losing the connection. This is important where authentication is using short-lived tokens as it allows a new token to be presented before the old token expires. I don't see any support for this in this library and it would be good to have so that we avoid the old "disconnect, reconnect" juggling.