Change state_information to return feature values (#804)

This changes `state_information` to return the names and values of
all defined features.
It was originally a "temporary" hack to show some extra, device-specific
information in the cli tool, but now that we have device-defined
features we can leverage them.

kasa/iot/iotplug.py

@@ -1,6 +1,6 @@
 """Module for smart plugs (HS100, HS110, ..)."""
 import logging
-from typing import Any, Dict, Optional
+from typing import Optional
 
 from ..device_type import DeviceType
 from ..deviceconfig import DeviceConfig
@@ -99,12 +99,6 @@ class IotPlug(IotDevice):
             "system", "set_led_off", {"off": int(not state)}
         )
 
-    @property  # type: ignore
-    @requires_update
-    def state_information(self) -> Dict[str, Any]:
-        """Return switch-specific state information."""
-        return {}
-
 
 class IotWallSwitch(IotPlug):
     """Representation of a TP-Link Smart Wall Switch."""