fix: #1398 add hint to FormBuilderDropdown

lib/src/fields/form_builder_dropdown.dart

@@ -218,6 +218,15 @@ class FormBuilderDropdown<T> extends FormBuilderFieldDecoration<T> {
   /// are defined by the corresponding properties of the [borderRadius].
   final BorderRadius? borderRadius;
 
+  /// A placeholder widget that is displayed by the dropdown button.
+  ///
+  /// If [value] is null and the dropdown is enabled ([items] and [onChanged] are non-null),
+  /// this widget is displayed as a placeholder for the dropdown button's value.
+  ///
+  /// If [value] is null and the dropdown is disabled and [disabledHint] is null,
+  /// this widget is used as the placeholder.
+  final Widget? hint;
+
   /// Creates field for Dropdown button
   FormBuilderDropdown({
     super.key,
@@ -253,6 +262,7 @@ class FormBuilderDropdown<T> extends FormBuilderFieldDecoration<T> {
     this.enableFeedback,
     this.borderRadius,
     this.alignment = AlignmentDirectional.centerStart,
+    this.hint,
   }) : super(
           builder: (FormFieldState<T?> field) {
             final state = field as _FormBuilderDropdownState<T>;
@@ -289,6 +299,7 @@ class FormBuilderDropdown<T> extends FormBuilderFieldDecoration<T> {
               borderRadius: borderRadius,
               enableFeedback: enableFeedback,
               alignment: alignment,
+              hint: hint,
             );
           },
         );