Hypothesis VCF strategies

[tomwhite]

It would be useful to use Hypothesis to generate VCF files for testing vcf_to_zarr and the forthcoming zarr_to_vcf (#924) functions.

There is already test code in sgkit for generating VCF files, but currently this is only used to generate a static VCF file. It should be possible to repurpose the generator to use Hypothesis, which excels at finding tests cases humans don't think of.

It might be worth putting this in its own project at some point as it should be standalone and may be generally useful. (@ravwojdyla suggested something similar in #23.)