[mqtt.homeassistant] Improve Cover support (#15875)

[mqtt.homeassistant] Improve Cover support (#15875)

* [mqtt.homeassistant] improve Cover support

 * Add support for covers that report position
 * Handle when command and state values for OPEN/CLOSE/STOP
   differ (as they do by default)
 * Expose the full cover state, since it can have tell you
   if the cover is moving or not
 * Handle covers that have a position only, but not a state

* add constants to clarify up/down values

* Be sure to parse percents from strings in RollshutterValue

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Signed-off-by: Cody Cutrer <cody@cutrer.us>