Detect offline hub children and fail fast on writes to them (#1830)

Hubs keep reporting for children they have lost contact with, so callers
see stale values as if they were live. Trying to write to such devices
fail after timeouts with a generic error.
This PR exposes the offline/online information as reported by the hub,
and refuses writes to unreachable child devices without waiting for
device errors.

In details:
- Add new API: `Device.is_available`; hub children report unavailable
when the hub marks them offline or no longer lists them.
- New feature `available` for child devices to make it visible to the
CLI.
- Refuse writes to unavailable children with the new
`DeviceUnavailableError` instead of waiting for the hub to time out.

kasa/device.py

@@ -277,6 +277,11 @@ class Device(ABC):
         """Return True if device is off."""
         return not self.is_on
 
+    @property
+    def is_available(self) -> bool:
+        """Return False if the device is known to be unreachable."""
+        return True
+
     @abstractmethod
     async def turn_on(self, **kwargs) -> dict:
         """Turn on the device."""