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/device.py

@@ -3,13 +3,14 @@ import logging
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from datetime import datetime
-from typing import Any, Dict, List, Optional, Sequence, Set, Union
+from typing import Any, Dict, List, Optional, Sequence, Union
 
 from .credentials import Credentials
 from .device_type import DeviceType
 from .deviceconfig import DeviceConfig
 from .emeterstatus import EmeterStatus
 from .exceptions import SmartDeviceException
+from .feature import Feature
 from .iotprotocol import IotProtocol
 from .protocol import BaseProtocol
 from .xortransport import XorTransport
@@ -69,6 +70,7 @@ class Device(ABC):
         self._discovery_info: Optional[Dict[str, Any]] = None
 
         self.modules: Dict[str, Any] = {}
+        self._features: Dict[str, Feature] = {}
 
     @staticmethod
     async def connect(
@@ -296,9 +298,16 @@ class Device(ABC):
         """Return the key state information."""
 
     @property
-    @abstractmethod
-    def features(self) -> Set[str]:
+    def features(self) -> Dict[str, Feature]:
         """Return the list of supported features."""
+        return self._features
+
+    def _add_feature(self, feature: Feature):
+        """Add a new feature to the device."""
+        desc_name = feature.name.lower().replace(" ", "_")
+        if desc_name in self._features:
+            raise SmartDeviceException("Duplicate feature name %s" % desc_name)
+        self._features[desc_name] = feature
 
     @property
     @abstractmethod