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/__init__.py

@@ -33,6 +33,7 @@ from kasa.exceptions import (
     TimeoutException,
     UnsupportedDeviceException,
 )
+from kasa.feature import Feature, FeatureType
 from kasa.iot.iotbulb import BulbPreset, TurnOnBehavior, TurnOnBehaviors
 from kasa.iotprotocol import (
     IotProtocol,
@@ -54,6 +55,8 @@ __all__ = [
     "TurnOnBehaviors",
     "TurnOnBehavior",
     "DeviceType",
+    "Feature",
+    "FeatureType",
     "EmeterStatus",
     "Device",
     "Bulb",