Added Styles support to PhoneFieldHint

[tehsunnliu]

Currently, the PhoneFieldHint widget only exposes decoration for customizing the TextField. This allows us to change placeholder/hint styling, but not the style of the actual user-entered text.

This is problematic in apps with dark themes:

• The placeholder (hintText) can be styled white,
• But the typed text still defaults to black, making it invisible against a dark background,

The workaround today is to supply a custom child: TextField, but doing so disables the automatic hint picker behavior of PhoneFieldHint, which defeats the purpose of using this widget.

This PR Adds an optional parameter to PhoneFieldHint:

• TextStyle? style → controls the style of the entered text