Add a MQTT sink

[phrrngtn]

Hi, this is a follow on from the excellent work you did on Automated Meter Reading (ERT/SCM/SCM+).

While looking at the issue of ingesting meter readings into a database backend, I notice there is some prior art on using rtlamr to get data into Home Assistant via MQTT: https://github.com/ragingcomputer/amridm2mqtt

I will do some experimentation with this but wanted to open this ticket as a placeholder.

thanks!