net: mqtt-sn: Add Gateway Advertisement and Discovery process support

Fixes: #78010
This commit implements the "Gateway Advertisement and Discovery" process
defined in section 6.1 of the MQTT-SN specification.
This includes breaking changes to the transport interface and the default
included UDP interface implementation as support for UDP multicast
messages is added as implemented by the Paho MQTT-SN Gateway.

Signed-off-by: Kenneth Witham <kennywitham4@gmail.com>