smartplugin: add pause_item (instead of suspend item)

[Morg42]

This item more or less replaces the suspend_item, but it hard-stops and re-runs the plugin instead of trying to suspend it while being active.

Modified and adjusten SmartPlugin and MqttPlugin as well as both sample plugins and their plugin.yaml files.

Works with adjusted zigbee2mqtt Plugin; plugin wishing to implement this need to

• be restartable (though metadata restartable is not queried)
• implement code snippets in init, run, stop, parse_item, update_item (see sample plugins)

Also cleaned up SmartPlugin, commented the reason for using class members and added some type hinting.

As far as I can see, the changes should be non-breaking.

[Morg42]

On merging, also merge smarthomeNG/plugins#945 to mitigate breaking changes to plugins using the old suspend_item mechanics.