Support for nulls_first / nulls_last in CursorPagination

[jonatanvm]

Django supports asc(nulls_first=None, nulls_last=None) and desc(nulls_first=None, nulls_last=None), but DRF does not.
Even if you write your own OrderingFilter which orders the results with nulls last, CursorPagination will override that in:

if reverse:
    queryset = queryset.order_by(*_reverse_ordering(self.ordering))
else:
    queryset = queryset.order_by(*self.ordering)

My suggestion is to add a configurable class variable that let's us configure whether nulls will be sorted first or last. Alternatively it would be helpful if ordering was extracted to a function that could be overridden.

[yangchoi]

I was also looking to discuss the same feature. If this feature is added, it will be a great help.