Fixes #125199

This PR is to add a new property `expandedInsets` so that the `DropdownMenu` can be expandable and has some margins around.

<details><summary>Example: Setting `expandedInsets` to `EdgeInsets.zero`</summary>

```dart
import 'package:flutter/material.dart';

void main() => runApp(const DropdownMenuExample());

class DropdownMenuExample extends StatefulWidget {
  const DropdownMenuExample({super.key});

  @override
  State<DropdownMenuExample> createState() => _DropdownMenuExampleState();
}

class _DropdownMenuExampleState extends State<DropdownMenuExample> {
  final TextEditingController colorController = TextEditingController();
  ColorLabel? selectedColor;

  @override
  Widget build(BuildContext context) {
    final List<DropdownMenuEntry<ColorLabel>> colorEntries = <DropdownMenuEntry<ColorLabel>>[];
    for (final ColorLabel color in ColorLabel.values) {
      colorEntries.add(
        DropdownMenuEntry<ColorLabel>(value: color, label: color.label, enabled: color.label != 'Grey'),
      );
    }

    return MaterialApp(
      theme: ThemeData(
        useMaterial3: true,
        colorSchemeSeed: Colors.green,
      ),
      home: Scaffold(
        body: Center(
          child: Container(
            width: 500,
            height: 500,
            color: Colors.orange,
            child: DropdownMenu<ColorLabel>(
              expandedInsets: EdgeInsets.zero,
              inputDecorationTheme: const InputDecorationTheme(
                filled: true,
                fillColor: Colors.white,
                border: OutlineInputBorder(),
              ),
              controller: colorController,
              dropdownMenuEntries: colorEntries,
              onSelected: (ColorLabel? color) {
                setState(() {
                  selectedColor = color;
                });
              },
              // expandedInsets: EdgeInsets.only(left: 35.0, right: 20.0, top: 80),
            ),
          ),
        ),
      ),
    );
  }
}

enum ColorLabel {
  blue('Blue', Colors.blue),
  pink('Pink', Colors.pink),
  green('Green', Colors.green),
  yellow('Yellow', Colors.yellow),
  grey('Grey', Colors.grey);

  const ColorLabel(this.label, this.color);
  final String label;
  final Color color;
}
```

<img width="500" alt="Screenshot 2023-06-28 at 11 33 20 PM" src="https://github.com/flutter/flutter/assets/36861262/e703f8a2-6e7c-45a0-86cf-d96da6dc157a">

</details>