Xiaomi Aqara: Remove/Add device service added (#10150)

* First draft of a remove device service. Fixes https://github.com/home-assistant/home-assistant/issues/9571.

* Add device service introduced. Enables the join permission of the gateway to pair a new sub device within the next 30 seconds.

* Schema validation added and some refactoring.

* A more precise validation of the gw_mac (ffffffffffff vs. ff:ff:ff:ff:ff:ff).

* Persistent notification added to provide some feedback.

* Pylint warning disabled. The methods are used indirectly.

* CODEOWNERS reference updated.