Add support for Add to button widget

We've introduced Add to feature in https://github.com/home-assistant/android/pull/5622 and it would be nice to extend it to Button widget.

The UI for selecting an action is available starting from HA core 2025.12.0 or (in nightly/dev).

- we need to define a new action in `EntityAddToAction` for Button widget
- we need to adjust the logic of EntityAddToHandler to add the action when it makes sense
- modify `ButtonWidgetConfigureActivity` to take the entity as parameter and prefill the selected entity

In this task you will also have to update the unit tests in EntityAddToHandler