Add basic mqtt support

[sfudeus]

This adds basic MQTT support.

When I started playing around with HomeAssistant I tried to find an easy way to integrate the data from the RCT inverter. Having this data already in prometheus, one option was to read it from prometheus - but this is not an ideal integration with HomeAssistant. Another option was an additional bridge between RCT and MQTT, using the rctclient directly. But each individual client against the inverter produces more interfering traffic, so the natural thing seemed to be to just let rctmon emit data to an additional backend. For the sake of simplicity, the MQTT topic names are just derived from the promtheus metric names. This does not always end in ideal or intuitive names, but it works well enough.