[go_router] Added `onPushRoute`; intercepts route pushes from the application host

[sarbagyastha]

Adds onPushRoute to the GoRouter.

GoRouter(
  ...
  onPushRoute: (RouteInformation routeInformation) {
    final location = routeInformation.location;

    if(location == '/prevent') return PushRouteDecision.prevent; // intercepts the route completely
    if(location == '/delegate') return PushRouteDecision.delegate; // delegates the route to other binding observers to handle
    return PushRouteDecision.navigate; // default; the router handles the route
  }
)

Fixes https://github.com/flutter/flutter/issues/103659 Fixes https://github.com/flutter/flutter/issues/121870

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