add list builder wrapper to support pulltorefresh

[Mikhail-Ivanou]

Add wrapper, so it's possible to use any widget as parent (RefreshIndicator for example)

Usage:
Widget build(BuildContext context) {
final today = DateTime.now().startOfDay();
return GroupedListView(
wrapper: (child) {
return RefreshIndicator(
onRefresh: () async {

        return Future.value();
      },
      child: child,
    );
  },
  elements: state.items,
  groupBy: (item) => item.createdAt.startOfDay(),
  groupItemBuilder: (context, element, first, last) => last....