[go_router] Add titleBuilder to GoRoute and GoRouterState

[getBoolean]

This is something I hacked together, bringing the titleBuilder from auto_route to go_router. I suspect it will need some changes, as I am not familiar with the internals of GoRouter.

• Issue: [go_router] ShellRoute needs a way to identify the current top route flutter#140297

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[chunhtai]

This seems to be a very limited use case, I think the root problem is that the shellroute doesn't have information about the current child. I think this is something we should figure out a better API to expose such thing. WDYT?

[getBoolean]

@chunhtai yes that would work, though I wasn't able to come up with a more generic solution without casting. Perhaps a solution could involve generics on the GoRoute class, but I'm not sure how that would be done without loosing the types in the routes list

[getBoolean]

I experimented with having adding generic type that the custom data builder can return. it is possible, but i had to add generics to nearly every go_router class. (Probably not what we want, but what are your thoughts?)
getBoolean@80a7022, see test GoRoute titleBuilder accessible from StatefulShellRoute for usage

The result is really good though, the titleBuilder can return any data type and access it from the ShellRoute.builder

[getBoolean]

In 0ddddb6, I converted the title builder to extensions, similar to Flutter's theme extensions

Usage:

GoRoute(
  path: 'b',
  extensions: <GoRouterStateExtensionBuilder>[
    (_, __) => const RouteTitleExtension('B'),
    (_, __) => const OtherCustomDataExtension('B.2'),
  ],
  builder: (BuildContext context, GoRouterState state) {
    return const Scaffold(
      body: Text('Screen 2'),
    );
  },
)

class RouteTitleExtension
    implements GoRouterStateExtension<RouteTitleExtension> {
  const RouteTitleExtension(this.title);

  final String title;
}

GoRouterState.of(context).extension<RouteTitleExtension>(context).title

[getBoolean]

New PR opened based on 13.0.0 #5736