[mqtt.homeassistant] Use a single channel for all scenes on a device

[ccutrer]

It accepts either object ID, or scene name (assuming the latter doesn't conflict with the former). Command descriptions are fully populated. You also no longer need to deal with sending the payload_on.

Merging components is refactored slightly, now that multiple component types do it.

Closes #18259

[ccutrer]

My item: String TheaterLights_Scene { channel="mqtt:homeassistant:mosquitto:zigbee2mqtt_5F1221051039810110150109113116116_5F14:scene" }

In MainUI:

In BasicUI, with this sitemap:

sitemap test label="Test" {
  Switch item=TheaterLights_Scene
}

So it's properly picking up the command description. From rules, I can send either "scene_1" or "House" as the command, and both get translated to the proper JSON when sent to MQTT.