Add seqr_loader conversion script for discussion

[MattWellie]

Fixes

• BCH and other seqr-using sites want to make use of pre-annotated data, bypassing documented Talos entrypoint
• Closes Documentation/examples for users starting from a matrix table (instead of a vcf) #516

Proposed Changes

• Adds a candidate script, based on the schema in Documentation/examples for users starting from a matrix table (instead of a vcf) #516, and assuming that's created using a comparable process to what we used internally
• extra handling of consequence_terms - in seqr_loader this is an array of Strings, in bcftools it's a single delimited String, plus the terms are slightly altered. This script maps the VEP terms to the BCFtools terms, and crushes the array into a &-delimited String.
• Still requires some attention on fields which were not explicitly noted in the Documentation/examples for users starting from a matrix table (instead of a vcf) #516 schema - gnomAD, AlphaMissense