Generalize smartdevice child support (#775)

* Initialize children's modules (and features) using the child component negotiation results
* Set device_type based on the device response
* Print out child features in cli 'state'
* Add --child option to cli 'command' to allow targeting child devices
* Guard "generic" features like rssi, ssid, etc. only to devices which have this information

Note, we do not currently perform queries on child modules so some data may not be available. At the moment, a stop-gap solution to use parent's data is used but this is not always correct; even if the device shares the same clock and cloud connectivity, it may have its own firmware updates.

kasa/device.py

@@ -3,7 +3,7 @@ import logging
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from datetime import datetime
-from typing import Any, Dict, List, Optional, Sequence, Union
+from typing import Any, Dict, List, Mapping, Optional, Sequence, Union
 
 from .credentials import Credentials
 from .device_type import DeviceType
@@ -71,6 +71,8 @@ class Device(ABC):
 
         self.modules: Dict[str, Any] = {}
         self._features: Dict[str, Feature] = {}
+        self._parent: Optional["Device"] = None
+        self._children: Mapping[str, "Device"] = {}
 
     @staticmethod
     async def connect(
@@ -182,9 +184,13 @@ class Device(ABC):
         return await self.protocol.query(request=request)
 
     @property
-    @abstractmethod
     def children(self) -> Sequence["Device"]:
         """Returns the child devices."""
+        return list(self._children.values())
+
+    def get_child_device(self, id_: str) -> "Device":
+        """Return child device by its ID."""
+        return self._children[id_]
 
     @property
     @abstractmethod