Bulbs and light strips can fade smoothly when their brightness or colour
changes, instead of snapping straight to the new value. The Tapo app
exposes this as "Smooth Transition". python-kasa cannot read or change
it today, so the setting is missing from Home Assistant even though the
device supports it.
This adds support for this feature as a switch:
- read `change_state` in `_post_update_hook`
- add a `change_transition` property and `set_change_transition()`
- register a `smooth_transition_change` switch feature when the device
reports the flag
