[mqtt.homeassistant] Support new "device" discovery

[ccutrer]

See https://www.home-assistant.io/integrations/mqtt/#device-discovery-payload. This is new in Home Assistant 2024.12, so it's unlikely (m)any devices are using it yet. For the most part it should be relatively simple to separate out the children components. We'll just need to make sure availability topics from the root of the JSON don't get lost.

I also don't think we need to worry about any of the transition messages HASS has to do, since items don't disappear when the discovery info does - just the channel. And then when they re-appear they automatically get re-linked.