Add VRS data to CSV output when post mapped HGVS strings are requested

[bencap]

Along with our post mapped HGVS strings, we should also provide a VRS digest of the post mapped variant to improve our support for VRS. We don't need to return this field with every request, but we should return it with every request that asks for mapped HGVS data.

We should:

• Add a field post_mapped_vrs_digest to the variant CSV export that holds the post mapped VRS digest corresponding to a mapped variant and the returned post mapped HGVS objects.