Don't materialize any data on `count_rows` if the filter can be satisfied by a scalar index

[westonpace]

If we can satisfy a filter with a scalar index then we should be able to implement a fast path for count_rows where we simply return the size of the allow_list or even possibly subtract the size of the block_list from the metadata-only count_rows (might be some tricky bits if there are deletion vectors) to quickly answer queries like count_rows("x != 'foo'").

[ighose]

Hi @westonpace, I would like to work on this. Could you please assign this to me?