consider supporting other output formats (csv, json, arrow?) in joint caller

[timodonnell]

There's quite a lot of business logic in VCFOutput around here where we take MultiSampleMultiAlleleEvidence instances and make them into htsjdk.VariantContext instances.

This makes it non-trivial to support other output formats like csv since that logic would have to get reproduced there.

It may make sense to make an intermediate representation for joint caller calls, which can then can converted either to htsjdk variant contexts for vcf output or other formats like csv.

[arahuja]

We could even support a cross-language serialization format

[timodonnell]

Yeah @hammer suggests json