Generalize assets to any block_type but using

AssetKey and the metadata store
PLAT-204

@cpennington @doctoryes plz review (hopefully I'll fix the 2 typos I just noticed before you get to it)
@doctoryes we need to add a story to create a filter kwarg on `get_all` or a separate fn to satisfy @nasthagiri 's request. I assume I shouldn't go ahead and add it, but perhaps I'll check the sort stuff to ensure it can handle the arbitrary fields?