Adds `FeatureType.Action` making it possible to expose features like
"reboot", "test alarm", "pair" etc.
The `attribute_getter` is no longer mandatory, but it will raise an
exception if not defined for other types than actions.
Trying to read returns a static string `<Action>`.
This overloads the `set_value` to call the given callable on any value.
This also fixes the `play` and `stop` coroutines of the alarm module to
await the call.
6e5cae1f47