Support for MQTT-SN

Some sensors live in a low power state, and only wake up when triggered (eg. https://github.com/brandond/esphome-tuya_pir ).

The overhead of connecting to the network, then connecting to MQTT and sending data can add quite a bit of latency, in the above example, there is around 3-4 seconds from when motion is detected to when an action is triggered by this.

The connection latency can be minimised by using fast_connect in the wifi settings, and a static IP, however, there is still significant overhead to establish the MQTT connection.

Supporting MQTT/SN (http://mqtt.org/tag/mqtt-sn) will eliminate the MQTT connection latency - the device can wake up, connect to the network, fire off it's UDP packets and go back to sleep, (optionally waiting for a response if QOS is set).

Add support for MQTT/SN.