feat: Add alignmentOffset to DropdownMenu (flutter#151731)

Changes:
- Introduced `alignmentOffset` property to `DropdownMenu` to allow adjustment of menu alignment.
- Updated the `MenuAnchor` widget to utilize the new `alignmentOffset` property.
- Default value for `alignmentOffset` is `Offset.zero`, meaning no additional spacing by default.

Motivation: 
- This PR closes flutter#151524 
- @nate-thegrate, please let me know if I need to make changes in order for this PR to be merged

The motivation behind this change is to provide developers with more control over the spacing between the `MenuAnchor` and `dropdownMenuEntries` in the `DropdownMenu` widget. This enhancement allows for better alignment and customization of the dropdown menu appearance.
| before | after |
| :---: | :---: |
| <img width="372" alt="Screenshot 2024-07-14 at 8 03 14�PM" src="https://github.com/user-attachments/assets/4a45b843-7fa4-44fd-843c-c7209c6f57ae">      |      <img width="364" alt="Screenshot 2024-07-14 at 8 03 27�PM" src="https://github.com/user-attachments/assets/12e8b857-aefb-4aaf-a310-4a002abcbc2f">   |

Initially, it was suggested to use a `SizedBox` to introduce spacing. However, upon further examination of the source code, it was discovered that the `DropdownMenuEntries` are rendered on the screen via an `OverlayPortal`. This necessitated leveraging the existing `alignmentOffset` property within the `MenuAnchor` for a more seamless and effective alignment adjustment.

Author: jasonkang14

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

@@ -175,6 +175,7 @@ class DropdownMenu<T> extends StatefulWidget {
     this.expandedInsets,
     this.filterCallback,
     this.searchCallback,
+    this.alignmentOffset,
     required this.dropdownMenuEntries,
     this.inputFormatters,
   }) : assert(filterCallback == null || enableFilter);
@@ -475,6 +476,9 @@ class DropdownMenu<T> extends StatefulWidget {
   ///    and notifies its listeners on [TextEditingValue] changes.
   final List<TextInputFormatter>? inputFormatters;
 
+  /// {@macro flutter.material.MenuAnchor.alignmentOffset}
+  final Offset? alignmentOffset;
+
   @override
   State<DropdownMenu<T>> createState() => _DropdownMenuState<T>();
 }
@@ -784,6 +788,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
 
     Widget menuAnchor = MenuAnchor(
       style: effectiveMenuStyle,
+      alignmentOffset: widget.alignmentOffset,
       controller: _controller,
       menuChildren: menu,
       crossAxisUnconstrained: false,

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

@@ -181,6 +181,7 @@ class MenuAnchor extends StatefulWidget {
   /// Defaults to the ambient [MenuThemeData.style].
   final MenuStyle? style;
 
+  /// {@template flutter.material.MenuAnchor.alignmentOffset}
   /// The offset of the menu relative to the alignment origin determined by
   /// [MenuStyle.alignment] on the [style] attribute and the ambient
   /// [Directionality].
@@ -201,6 +202,7 @@ class MenuAnchor extends StatefulWidget {
   /// [alignmentOffset] move the menu position to the left.
   ///
   /// Defaults to [Offset.zero].
+  /// {@endtemplate}
   final Offset? alignmentOffset;
 
   /// {@macro flutter.material.Material.clipBehavior}

packages/flutter/test/material/dropdown_menu_test.dart

@@ -2432,6 +2432,28 @@ void main() {
     final TextField textField = tester.widget(find.byType(TextField));
     expect(textField.keyboardType, TextInputType.text);
   });
+
+  testWidgets('DropdownMenu passes an alignmentOffset to MenuAnchor', (WidgetTester tester) async {
+    const Offset alignmentOffset = Offset(0, 16);
+
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<String>(
+            alignmentOffset: alignmentOffset,
+            dropdownMenuEntries: <DropdownMenuEntry<String>>[
+              DropdownMenuEntry<String>(value: '1', label: 'One'),
+              DropdownMenuEntry<String>(value: '2', label: 'Two'),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final MenuAnchor menuAnchor = tester.widget<MenuAnchor>(find.byType(MenuAnchor));
+
+    expect(menuAnchor.alignmentOffset, alignmentOffset);
+  });
 }
 
 enum TestMenu {