Add DropdownMenu.focusNode (#142516)
fixes [`DropdownMenu` doesn't have a focusNode](https://github.com/flutter/flutter/issues/142384)
### Code sample
<details>
<summary>expand to view the code sample</summary>
```dart
import 'package:flutter/material.dart';
enum TShirtSize {
s('S'),
m('M'),
l('L'),
xl('XL'),
xxl('XXL'),
;
const TShirtSize(this.label);
final String label;
}
void main() => runApp(const MyApp());
class MyApp extends StatefulWidget {
const MyApp({super.key});
@override
State<MyApp> createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
final FocusNode _focusNode = FocusNode();
@override
void dispose() {
_focusNode.dispose();
super.dispose();
}
@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: Scaffold(
appBar: AppBar(
title: const Text('DropdownMenu Sample'),
),
body: Center(
child: DropdownMenu<TShirtSize>(
focusNode: _focusNode,
initialSelection: TShirtSize.m,
label: const Text('T-Shirt Size'),
dropdownMenuEntries: TShirtSize.values.map((e) {
return DropdownMenuEntry<TShirtSize>(
value: e,
label: e.label,
);
}).toList(),
),
),
floatingActionButton: FloatingActionButton.extended(
onPressed: () {
_focusNode.requestFocus();
},
label: const Text('Request Focus on DropdownMenu'),
),
),
);
}
}
```
</details>
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