Add support for Add to button widget

[TimoPtr]

We've introduced Add to feature in #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