Support bulk insertion for `add_allele_frequencies`

[korikuzma]

Description

IMO we should be planning on bulk insertion from the jump here. I don't think we need to necessarily have the highest-powered insert method ever right away, but we could at least start with a plural rather than singular function signature

Originally posted by @jsstevenson in #28 (comment)

Use case

We want to be able to support bulk inserts when ingesting allele frequency data from VCF files