Add generic interface for accessing device features (#741)

This adds a generic interface for all device classes to introspect available device features,
that is necessary to make it easier to support a wide variety of supported devices with different set of features.
This will allow constructing generic interfaces (e.g., in homeassistant) that fetch and change these features without hard-coding the API calls.

`Device.features()` now returns a mapping of `<identifier, Feature>` where the `Feature` contains all necessary information (like the name, the icon, a way to get and change the setting) to present and change the defined feature through its interface.

kasa/iot/iotplug.py

@@ -4,6 +4,7 @@ from typing import Any, Dict, Optional
 
 from ..device_type import DeviceType
 from ..deviceconfig import DeviceConfig
+from ..feature import Feature, FeatureType
 from ..protocol import BaseProtocol
 from .iotdevice import IotDevice, requires_update
 from .modules import Antitheft, Cloud, Schedule, Time, Usage
@@ -56,6 +57,17 @@ class IotPlug(IotDevice):
         self.add_module("time", Time(self, "time"))
         self.add_module("cloud", Cloud(self, "cnCloud"))
 
+        self._add_feature(
+            Feature(
+                device=self,
+                name="LED",
+                icon="mdi:led-{state}",
+                attribute_getter="led",
+                attribute_setter="set_led",
+                type=FeatureType.Switch,
+            )
+        )
+
     @property  # type: ignore
     @requires_update
     def is_on(self) -> bool:
@@ -88,5 +100,4 @@ class IotPlug(IotDevice):
     @requires_update
     def state_information(self) -> Dict[str, Any]:
         """Return switch-specific state information."""
-        info = {"LED state": self.led, "On since": self.on_since}
-        return info
+        return {}