add option "target2current_temp" for aircondition devices

[Jack-Works]

this new option "target2current_temp" enables the following behavior:

• if the air conditioner is on, report target_temperature as current_temperature.
• if it is off, fallback to either bind_sensor or current_temp_prop.

this PR supports the following use case:

• expect HomeKit displays the target temperature instead and
• there is no sensor available in the room where the air conditioner works (therefore bind_sensor binds to a sensor in another room or the weather report sensor). If you have a sensor in the room, you don't need this option, just use bind_sensor.

with this option enabled, HomeKit reports the target temperature when it's on, and reports the sensor's temperature/temperature from the weather report when the air condition is off.

[al-one]

@Jack-Works Good idea! It's just that the option name report_target_temp_as_current_temp is too long. Can it be simplified?

[Jack-Works]

@Jack-Works Good idea! It's just that the option name report_target_temp_as_current_temp is too long. Can it be simplified?

I think that name reflects what it does. Can you think of any shorter name that still explain the functionality?

[al-one]

We have a similar option called target2current_position to cover entities, maybe this option could be named target2current_temp.