Support `bcftools`-style filtering

[tomwhite]

It would be very useful to have the ability to subset a dataset using bcftools expressions for regions (-r/-t), filter expressions (-i/-e), and samples (-s).

This is now possible using the work we have done in vcztools, such as https://github.com/sgkit-dev/vcztools/pull/234. In particular, we now have an index that allows efficient region queries (which will address #25).

This will also address #1084.

See an initial prototype in https://github.com/sgkit-dev/bio2zarr/issues/365#issuecomment-2859114862