Merge zigbee.mqtt plugin/backend and zwave.mqtt plugin/backend
The Zigbee and Z-Wave integrations currently require two tightly coupled pieces (the plugin and the backend) in order to work properly, for historical reasons (traditionally, Platypush plugins were stateless, while backends were the only way to get a listener to run in the background).
Since this is no longer the case (Platypush now provides the
RunnablePlugin object), it makes sense to simplify the configuration and the architecture and merge the plugin and the backend.
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