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`
