Add support for Fully's MQTT API

[cgarwood]

Fully Kiosk Browser can connect to an MQTT server and will publish its full device information every 60 seconds (same info we get from the REST API). It will also publish a variety of events on the MQTT server as they happen (motion detection, ac adapter plugged/unplugged, screensaver start/stop, device movement, etc). We should add MQTT support to this lib alongside the REST API support so we can add triggers based on events.

[crowbarz]

One major advantage of supporting MQTT is that the MQTT connection does not require the device's IP address to be hardcoded in config, rather the MQTT server's IP address is hardcoded on the device running Fully. If the device can connect to the network via multiple different network interfaces, eg. Ethernet or WiFi, chances are that each of the interfaces will have a different IP address, so being only able to specify a single IP address for a device in this library's associated HA integration would also prevent the device from being reachable if its primary network interface happens to be unavailable and it has failed over to another network interface.

So ideally, the MQTT integration would pick up the device's current IP address via the MQTT device update and use that to talk to Fully's REST interface, rather than having to specify the device's IP address manually when setting up the HA integration.

[Sholofly]

@cgarwood is there a way to contribute to this feature?

[Sholofly]

@cgarwood Can you review and, if possible, complete my PR on this?