MQTTPublish and Home Assistant MQTT Device Discovery

[bell...@gmail.com]

Primarily as a learning exercise, I have added support for Home Assistant MQTT Device Discovery to MQTTPublish. This is very early alpha code, but if you are interested in testing, influencing its future, etc; I have started a GitHub discussion here, https://github.com/weewx-mqtt/publish/discussions/19.

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