Add DropdownMenu.restorationId (flutter#166684)

## Description

This PR introduces `DropdownMenu.restorationId`.
This value is passed to the inner `TextField.restorationId` which is
required to activate the TextField restoration capability.

## Related Issue

Required for flutter#163721

## Tests

Adds 1 test.

Author: bleroux

packages/flutter/lib/src/material/dropdown_menu.dart

@@ -190,6 +190,7 @@ class DropdownMenu<T> extends StatefulWidget {
     this.closeBehavior = DropdownMenuCloseBehavior.all,
     this.maxLines = 1,
     this.textInputAction,
+    this.restorationId,
   }) : assert(filterCallback == null || enableFilter);
 
   /// Determine if the [DropdownMenu] is enabled.
@@ -527,6 +528,9 @@ class DropdownMenu<T> extends StatefulWidget {
   /// {@macro flutter.widgets.TextField.textInputAction}
   final TextInputAction? textInputAction;
 
+  /// {@macro flutter.material.textfield.restorationId}
+  final String? restorationId;
+
   @override
   State<DropdownMenu<T>> createState() => _DropdownMenuState<T>();
 }
@@ -1070,6 +1074,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
                     : null,
             suffixIcon: trailingButton,
           ).applyDefaults(effectiveInputDecorationTheme),
+          restorationId: widget.restorationId,
         );
 
         // If [expandedInsets] is not null, the width of the text field should depend

packages/flutter/test/material/dropdown_menu_test.dart

@@ -4213,6 +4213,27 @@ void main() {
 
     semantics.dispose();
   });
+
+  testWidgets('restorationId is passed to inner TextField', (WidgetTester tester) async {
+    const String restorationId = 'dropdown_menu';
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<TestMenu>(
+            dropdownMenuEntries: menuChildren,
+            requestFocusOnTap: true,
+            restorationId: restorationId,
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(TextField), findsOne);
+
+    final TextField textField = tester.firstWidget(find.byType(TextField));
+    expect(textField.restorationId, restorationId);
+  });
 }
 
 enum TestMenu {