FAB builder in `AutoTabsScaffold`

[emme1444]

Support building the FloatingActionButton based on router changes in AutoTabsScaffold. Much like the AppBar and BottomNavigationBar, etc..

AutoTabsScaffold(
  routes: [...],
  appBarBuilder: (context, tabsRouter) => ...,
  bottomNavigationBuilder: (_, tabsRouter) => ...,
  floatingActionButtonBuilder: (context, tabsRouter) {
    switch (tabsRouter.current.name) {
      case MyRoute.name:
        return FloatingActionButton(
          child: const Icon(Icons.add),
          onPressed: () {},
        );
      default:
        return null;
    }
  },
)

PR is coming.

Related: #545