[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 flutter/flutter#103659
Fixes flutter/flutter#121870

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[chunhtai]

This API currently can't differentiate between deeplink or browser backward and forward button. There are some efforts to send host information along with RouteInformation in case of deeplinks.
flutter/flutter#100624

I intend to close this one since this may be superseded once we start to handle host information.