Support SliverAppBar as header

[tlvenn]

I would like to use the SliverAppBar as the header but right now this is not possible because of the explicit SliverToBoxAdapter being used with the header.

https://github.com/TheMisir/flutter-listutils/blob/master/lib/src/custom_list_view.dart#L367

Ideally we should either detect if the header is an instance of theSliverAppBar or maybe more broadly let the user tell us if the header is a sliver (this concept could / should probably be extended to the footer as well if we decide to go this way).

@themisir what do you think ?