Auto auto-off module for smartdevice (#760)

Adds auto-off implementation. The feature stays enabled after the timer runs out, and it will start the countdown if the device is turned on again without explicitly disabling it.

New features:
* Switch to select if enabled: `Auto off enabled (auto_off_enabled): False`
* Setting to change the delay: `Auto off minutes (auto_off_minutes): 222`
* If timer is active, datetime object when the device gets turned off: `Auto off at (auto_off_at): None`

kasa/tests/fakeprotocol_smart.py

@@ -46,6 +46,7 @@ class FakeSmartTransport(BaseTransport):
 
     FIXTURE_MISSING_MAP = {
         "get_wireless_scan_info": ("wireless", {"ap_list": [], "wep_supported": False}),
+        "get_auto_off_config": ("auto_off", {'delay_min': 10, 'enable': False}),
         "get_led_info": (
             "led",
             {