gw: Add decoding KNOT register command
gw: Add decoding KNOT register command

This patch adds the initial code in the GW core to decode and call
the 'signup' callback implemented by the IoT protocol driver.

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source