Repository: flutter
Issue: [go_router_builder]: Specify observers to the ShellRouteData
Labels: package,has reproducible steps,P3,p: go_router_builder,found in release: 3.10,team-go_router,triaged-go_router,found in release: 3.14

### Is there an existing issue for this?

- [X] I have searched the [existing issues](https://github.com/flutter/flutter/issues)
- [X] I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)

### Steps to reproduce

1. Clone a this oficial example:
https://github.com/flutter/packages/blob/main/packages/go_router_builder/example/lib/main.dart
or
https://github.com/flutter/packages/blob/main/packages/go_router_builder/example/lib/shell_route_example.dart

2. Add a custom observer to `GoRouter`
```dart
GoRouter(
  observers: [
    GoRouterObserver(),
  ],
...
);
```


### Expected results

View the logs as you navigate through the different routes

### Actual results

Only see one log

### Code sample

<details><summary>Code sample</summary>

```dart

class GoRouterObserver extends NavigatorObserver {
  @override
  void didPush(Route<dynamic> route, Route<dynamic>? previousRoute) {
    super.didPush(route, previousRoute);
    log(
      'GoRouterObserver: (didPush) new=${route.settings.name} '
      'previousRoute= ${previousRoute?.settings.name}',
    );
  }

  @override
  void didPop(Route<dynamic> route, Route<dynamic>? previousRoute) {
    super.didPop(route, previousRoute);
    log(
      'GoRouterObserver: (didPop) new=${route.settings.name} '
      'previousRoute= ${previousRoute?.settings.name}',
    );
  }

  @override
  void didRemove(Route<dynamic> route, Route<dynamic>? previousRoute) {
    super.didRemove(route, previousRoute);
    log(
      'GoRouterObserver: (didRemove) new=${route.settings.name} '
      'previousRoute= ${previousRoute?.settings.name}',
    );
  }

  @override
  void didReplace({Route<dynamic>? newRoute, Route<dynamic>? oldRoute}) {
    super.didReplace(newRoute: newRoute, oldRoute: oldRoute);
    log(
      'GoRouterObserver: (didReplace) new=${newRoute?.settings.name}, '
      'old=${oldRoute?.settings.name}',
    );
  }
}
```

</details>


### Screenshots or Video

_No response_

### Logs

<details><summary>Logs</summary>

```console
[Paste your logs here]
```

</details>


### Flutter Doctor output

<details><summary>Doctor output</summary>

```console
[✓] Flutter (Channel stable, 3.10.6, on macOS 13.1 22C65 darwin-arm64, locale es-US)
    • Flutter version 3.10.6 on channel stable at /Users/yeikel/fvm/versions/3.10.6
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision f468f3366c (hace 3 semanas), 2023-07-12 15:19:05 -0700
    • Engine revision cdbeda788a
    • Dart version 3.0.6
    • DevTools version 2.23.1

[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
    • Android SDK at /Users/yeikel/Library/Android/sdk/
    • Platform android-33, build-tools 33.0.1
    • ANDROID_HOME = /Users/yeikel/Library/Android/sdk/
    • Java binary at: /Applications/Android Studio 2.app/Contents/jbr/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-17.0.6b802.4-9586694)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 14.2)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 14C18
    • CocoaPods version 1.12.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
    • Android Studio at /Applications/Android Studio 2020.3.1.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)

[✓] Android Studio (version 2021.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)

[✓] Android Studio (version 2022.2)
    • Android Studio at /Applications/Android Studio 2.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-17.0.6b802.4-9586694)

[✓] VS Code (version 1.78.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.70.0

[✓] Connected device (3 available)
    • macOS (desktop)           • macos                     • darwin-arm64   • macOS 13.1 22C65 darwin-arm64
    • Chrome (web)              • chrome                    • web-javascript • Google Chrome 115.0.5790.170

[✓] Network resources
    • All expected network resources are available.

• No issues found!
```

</details>