Create MQTT endpoint for receiving data

[aufdenkampe]

As mentioned in #95, and related to our long-term interest in creating a lighter-weight data posting system (#11), we might consider created an Message Queuing Telemetry Transport (MQTT) API endpoint as an alternate to our current HTTP RESTful POST API (which we should always support for backward compatibility).

MQTT has gained a lot of traction over the last year or two, and is being used by many/most IoT systems, such https://thingspeak.com.

MQTT "works on top of the TCP/IP protocol", and is "designed for connections with remote locations where a "small code footprint" is required or the network bandwidth is limited".

MQTT is fully supported by the TinyGSM library that we use for radio communications. Arduino code examples for MQTT abound, so supporting it on the device-side will be relatively straightforward.