feat(firebase_ui_auth): Add providerBuilder for sign in and register screen layout customization

[fabiocarneiro]

This PR introduces a major flexibility enhancement for SignInScreen, RegisterScreen, and LoginView.

It adds a providersBuilder callback that gives developers full control over the layout, ordering, and grouping of provider buttons.

Before

Previously, you could only provide a list of providers, and they would be rendered in a fixed order (Email -> Phone -> Email Link -> OAuth) regardless of the order they were passed in the providers list.

SignInScreen(
  providers: [
    EmailAuthProvider(),
    GoogleProvider(clientId: '...'),
    AppleProvider(),
  ],
)

After

With the new optional providersBuilder, you can fully customize the layout, grouping, and ordering of the provider buttons.

SignInScreen(
  providers: [
    EmailAuthProvider(),
    GoogleProvider(clientId: '...'),
    AppleProvider(),
  ],
  // New: Custom layout builder
  providersBuilder: (context, providers, content) {
    return Column(
      children: [
        // Render the Email provider first
        providers.firstWhere((p) => p is EmailAuthProvider),

        const Padding(
          padding: EdgeInsets.symmetric(vertical: 16),
          child: Divider(child: Text("Or sign in with")),
        ),

        // Grouping OAuth providers horizontally
        Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            providers.firstWhere((p) => p is GoogleProvider),
            providers.firstWhere((p) => p is AppleProvider),
          ],
        ),
      ],
    );
  },
)

💉 Dependency Injection Support

• Added FirebaseAuthProvider to facilitate better dependency injection of the FirebaseAuth instance throughout the widget tree, simplifying how auth state is accessed in child components.

📦 Changes

• Added providersBuilder property to SignInScreen, RegisterScreen, and LoginView.
• Refactored LoginView to use a default builder that preserves legacy behavior (fixed order) if no custom builder is provided.
• Updated OAuth provider buttons to ensure they remain compatible with the new flexible layout system.
• Added missing dependencies to OAuth packages to support the new architecture

Related Issues

None

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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

[gemini-code-assist]

Code Review

This pull request introduces FirebaseAuthProvider, an InheritedWidget for managing FirebaseAuth instances throughout the widget tree, and adds a providersBuilder to allow custom ordering and appearance of authentication providers. Feedback identifies a stale configuration issue in the Google sign-in button and regressions in OAuth buttons where custom callbacks and tap actions are currently bypassed. Additionally, it is recommended to use FlutterError for better debugging and to clarify the documentation regarding the ProvidersBuilder implementation.