.vcf.bgz as a valid vcf extension in FileExtensions

[lindenb]

Description

VCF suppliers like gnomad, provide VCF with the vcf.bgz extension (eg https://storage.googleapis.com/gcp-public-data--gnomad/release/4.1/vcf/exomes/gnomad.exomes.v4.1.sites.chr1.vcf.bgz ) . This PR adds vcf.bgz as a valid extension, defined as COMPRESSED_VCF_BGZ

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[lbergelson]

👍 Looks like an improvement to me. My only comment is that we might want to make it a method "hasCompressedVcfExtension()" or something like that. We can always revisit that in the future though.