feat: use bulk methods for loading alleles in AnyVar

[jsstevenson]

As of biocommons/anyvar#369, AnyVar now supports bulk additions via HTTP and via Python API, so this PR refactors each respective AnyVar client to transmit variations in bulk.

• I performed a very unscientific experiment (a single VCF) and it seemed to be something like 20-30% faster in that specific case? It looks like the main remaining bottleneck is variant translation, although bulk inserts to the CAF table will be nice too
• Also did some cleanup in the HTTP client to move some repeated code into the shared method and move some request-specific code up into the respective functions