Support `ClimateEntityFeature.TURN_ON`

[mxr]

This issue summarizes discussion in other issues & can be used as a reference.

Once 2025.1 is live then the _enable_turn_on_off_backwards_compatibility functionality will cease to work and this integration will no longer support the climate.turn_on action.

To implement "turn on" we need to

• Add ClimateEntityFeature.TURN_ON to self._supported_features here.
• Implement a turn_on function (similar to turn_off here).
• Update the README

The body of turn_on should turn the unit to the last active HVAC mode. One way to store the last mode is in a small json file. Some other ways could be to create a sensor that stores the last mode or user-configurable select.