fixes [Long `FlexibleSpaceBar.title` doesn't respect the leading widget 
](https://github.com/flutter/flutter/issues/132030)

### Description

- This adds `FlexibleSpaceBarSettings.hasLeading` for the `FlexibleSpaceBar`'s title to respect the leading widget.
- Use the new `FlexibleSpaceBarSettings.hasLeading` property in the  `SliverAppBar` for its `FlexibleSpaceBar`.

### Code sample

<details> 
<summary>expand to view the code sample</summary> 

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

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        useMaterial3: true,
        brightness: Brightness.dark,
      ),
      home: const Example(),
    );
  }
}

class Example extends StatelessWidget {
  const Example({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          const Text('TargetPlatform.Android'),
          Theme(
            data: Theme.of(context).copyWith(
              platform: TargetPlatform.android,
            ),
            child: Container(
              height: 250,
              padding: const EdgeInsets.all(8),
              decoration: BoxDecoration(
                border: Border.all(
                  color: Colors.amber,
                  width: 4,
                ),
              ),
              child: const AppBarLeading(
                showLeading: true,
                showTitle: false,
              ),
            ),
          ),
          const Text('TargetPlatform.iOS'),
          Theme(
            data: Theme.of(context).copyWith(
              platform: TargetPlatform.iOS,
            ),
            child: Container(
              height: 250,
              padding: const EdgeInsets.all(8),
              decoration: BoxDecoration(
                border: Border.all(
                  color: Colors.amber,
                  width: 2,
                ),
              ),
              child: const AppBarLeading(
                showLeading: true,
                showTitle: false,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

class AppBarLeading extends StatelessWidget {
  const AppBarLeading({
    super.key,
    required this.showLeading,
    required this.showTitle,
  });

  final bool showLeading;
  final bool showTitle;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      drawer: const Drawer(),
      body: CustomScrollView(
        slivers: [
          SliverAppBar(
            automaticallyImplyLeading: showLeading,
            iconTheme: const IconThemeData(
              color: Colors.amber,
            ),
            title: showTitle ? const Text('AppBar') : null,
            flexibleSpace: FlexibleSpaceBar(
              title: Text('Title ' * 15),
              // centerTitle: true,
            ),
            toolbarHeight: showTitle ? 170 : 100,
          ),
        ],
      ),
    );
  }
}
``` 
	
</details>

### Before 

![Screenshot 2023-08-15 at 18 11 34](https://github.com/flutter/flutter/assets/48603081/4b798998-8549-43aa-b564-933ea14f494c)

### After

![Screenshot 2023-08-15 at 18 11 45](https://github.com/flutter/flutter/assets/48603081/b085a33a-db7d-40d4-8a12-ee37197b5bd4)