Add state feature for iot devices (#924)

This is allows a generic implementation for the switch platform in the
homeassistant integration.

Also elevates set_state(bool) to be part of the standard API.

kasa/device.py

@@ -138,6 +138,14 @@ class Device(ABC):
     async def turn_off(self, **kwargs) -> dict | None:
         """Turn off the device."""
 
+    @abstractmethod
+    async def set_state(self, on: bool):
+        """Set the device state to *on*.
+
+        This allows turning the device on and off.
+        See also *turn_off* and *turn_on*.
+        """
+
     @property
     def host(self) -> str:
         """The device host."""