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/feature.py

@@ -0,0 +1,50 @@
+"""Generic interface for defining device features."""
+from dataclasses import dataclass
+from enum import Enum, auto
+from typing import TYPE_CHECKING, Any, Callable, Optional, Union
+
+if TYPE_CHECKING:
+    from .device import Device
+
+
+class FeatureType(Enum):
+    """Type to help decide how to present the feature."""
+
+    Sensor = auto()
+    BinarySensor = auto()
+    Switch = auto()
+    Button = auto()
+
+
+@dataclass
+class Feature:
+    """Feature defines a generic interface for device features."""
+
+    #: Device instance required for getting and setting values
+    device: "Device"
+    #: User-friendly short description
+    name: str
+    #: Name of the property that allows accessing the value
+    attribute_getter: Union[str, Callable]
+    #: Name of the method that allows changing the value
+    attribute_setter: Optional[str] = None
+    #: Container storing the data, this overrides 'device' for getters
+    container: Any = None
+    #: Icon suggestion
+    icon: Optional[str] = None
+    #: Type of the feature
+    type: FeatureType = FeatureType.Sensor
+
+    @property
+    def value(self):
+        """Return the current value."""
+        container = self.container if self.container is not None else self.device
+        if isinstance(self.attribute_getter, Callable):
+            return self.attribute_getter(container)
+        return getattr(container, self.attribute_getter)
+
+    async def set_value(self, value):
+        """Set the value."""
+        if self.attribute_setter is None:
+            raise ValueError("Tried to set read-only feature.")
+        return await getattr(self.device, self.attribute_setter)(value)