Added util functions for dialogs

Use utility function showCountryPickerSheet to show a bottom sheet picker.

void _showCountryPicker() async{
    final country = await showCountryPickerSheet(context,);
    if (country != null) {
      setState(() {
        _selectedCountry = country;
      });
    }
}

Use utility function showCountryPickerDialog to show a dialog.

void _showCountryPicker() async{
    final country = await showCountryPickerDialog(context,);
    if (country != null) {
      setState(() {
        _selectedCountry = country;
      });
    }
}