Transform `button` to a Builder callback

[koukibadr]

Use case

Proposed Changes:

The button property will evolve from a Widget to a (context) -> Widget

Benefits:

• Dynamic Content: Allow the button to change based on the currently selected value or other internal picker state.
• Custom Layouts: Enable developers to include custom widgets, icons, or complex layouts directly within the button, not just plain text.
• Improved User Experience: Facilitate more context-aware and visually engaging picker interfaces.

Proposal

No response