Add MQTT Home Assistant publishing

[clinta]

Thanks for building and maintaining such a helpful project. rdc-proxy has been very useful, and this PR adds an optional MQTT path so decoded generator telemetry can be ingested by Home Assistant.

Summary:

• Add opt-in MQTT configuration and the paho-mqtt dependency.
• Publish retained telemetry topics under rdc_proxy/.
• Publish Home Assistant MQTT discovery configs grouped as a Generator device.
• Use the generator serial number for Home Assistant device/entity identity when available.
• Convert the RDC timestamp into a Home Assistant timestamp payload with timezone information.
• Add tests for config merging, publisher startup, discovery payloads, availability, and timestamp formatting.

Note:

• This branch also includes the existing listener-address config commit, which adds configurable web/proxy listen addresses.

Tests:

• .venv/bin/python -m pytest