support MQTT discovery for HomeAssistant

[faceprint]

Setting up the MQTT integration with HomeAssistant can be tedious. HomeAssistant supports a MQTT discovery protocol, described here:

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

teslamate should publish homeassistant discovery info, so sensors could be automatically created in HomeAssistant

[nebhale]

Now implemented: https://github.com/teslamate-org/teslamate/releases#release-v4.1.0