[iOS] Fix VoiceOver focus not shifting to Picker/DatePicker/TimePicker popups

[jsuarezruiz]

Description of Change

This PR include some extension methods to use UIAccessibility.PostNotification calls to notify VoiceOver when picker popups appear and close for Picker, DatePicker, and TimePicker.

• Open Picker: Posts a VoiceOver notification when an input view becomes visible.
• Close Picker: Posts a VoiceOver notification to return focus to the original control.

Based on the issue description, the picker should:

• When the Picker is opened, UIAccessibility.PostNotification(ScreenChanged, inputView) is called with the UIPickerView, VoiceOver focuses on the picker control.
• Once focused on the UIPickerView, VoiceOver users can naturally navigate through the individual items. This try to follow iOS accessibility patterns where you first focus on the container control, then navigate within it.
• When closing the picker, restores the VoiceOver focus.

Draft PR until include some related tests.

Issues Fixed

Fixes #30746