feat: support pause kraken pages when navigator changes.

[andycall]

支持通过 Flutter 提供的 RouteObserver 监听页面的跳转状态。如果页面被跳转到新页面，timer 和 requestAnimationCallback 会被暂停，其他状态都会被保留。当页面再次返回时，恢复先前的回调和状态。

// Register the RouteObserver as a navigation observer.
final RouteObserver<ModalRoute<void>> routeObserver = RouteObserver<ModalRoute<void>>();

void main() {
  KrakenWebsocket.initialize();

  runApp(MaterialApp(
    home: MyApp(),
    navigatorObservers: [routeObserver],
  ));
}


class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Second Route'),
      ),
      body: Center(
        child: Column(children: [
          Kraken(
            bundlePath: 'https://kraken.oss-cn-hangzhou.aliyuncs.com/demo/feeds-demo.js', 
            viewportHeight: 200, 
            routeObserver: routeObserver
          ),
      ),
    );
  }
}

[andycall]

LGTM

[wssgcg1213]