net: mqtt-sn: Add MQTT-SN library

This commit adds an implementation of MQTT-SN v1.2.
The specification is available on oasis-open.org:
https://www.oasis-open.org/committees/download.php/66091/MQTT-SN_spec_v1.2.pdf



The following things are missing in this implementation:
- Pre-defined topic IDs
- QoS -1 - it's most useful with predefined topics
- Gateway discovery using ADVERTISE, SEARCHGW and GWINFO messages.
- Setting the will topic and message after the initial connect
- Forwarder Encapsulation

Signed-off-by: René Beckmann <rene.beckmann@grandcentrix.net>