Add home-assistant discovery topics

[volca02]

After stabilising what should be in the JSON for the HVAC devices, add a templated discovery topic with every client as they are first discovered, and publish this after every reconnect.

https://www.home-assistant.io/docs/mqtt/discovery/

[volca02]

It would be much easier if such a string would be pre-formatted, with some special characters to be replaced with address. For that, we would be better off using fixed 2 characters for MQTT topic for the rest of the system, so that we don't have to erase any characters when composing such a string. But even now, we know we have at most 2 characters for the address part of mqtt topic, so such thing can be pre-prepared and a loop can be used to copy characters