Add deconz option to disable automatic addition of new devices (#40545)

* Allow disabling automatic additions of new devices from deconz

* Fix black

* Fix review comment

* Remove assertion

* Verify entity registry is empty

tests/components/deconz/test_gateway.py

@@ -81,6 +81,7 @@ async def test_gateway_setup(hass):
         assert gateway.master is True
         assert gateway.option_allow_clip_sensor is False
         assert gateway.option_allow_deconz_groups is True
+        assert gateway.option_allow_new_devices is True
 
         assert len(gateway.deconz_ids) == 0
         assert len(hass.states.async_all()) == 0