[go_router] Support for top level onEnter callback.

[omar-hanafy]

Description:
This PR introduces top level onEnter callback in RouteConfiguration to allow developers to intercept and conditionally block navigation based on incoming routes. It adds an example (top_level_on_enter.dart) demonstrating its usage, such as handling referral code from /referral.

What This PR Changes:

• Adds the onEnter callback (typedef OnEnter) to intercept route navigation before processing.
• Implements logic for onEnter in GoRouteInformationParser.
• Updates RouteConfiguration to include the onEnter callback.
• Adds a new example, top_level_on_enter.dart, to demonstrate the feature.
• Adds a test case to verify the onEnter callback functionality.

Simple usage example:

final GoRouter router = GoRouter(
  onEnter: (BuildContext context, Uri uri) {
    // Save the referral code (if provided) and block navigation to /referral.
    if (uri.path == '/referral') {
      saveReferralCode(context, uri.queryParameters['code']);
      return false;
    }

    return true; // Allow navigation for all other routes.
  },
  routes: [ ... ],
);

Impact:
Enables developers to implement route-specific logic, such as support for handling action-based deep links without navigation (160602)

Pre-launch Checklist:

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g., [go_router].
• I [linked to at least one issue that this PR fixes] in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or this PR is [exempt from CHANGELOG changes].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.