First commit of the matchmaker APIs

This is a wip translation of the matchmaker APIs into avro.

Autogenerated docs are here: http://cassiedoll.github.io/matchmaker-staging.html
To avoid ambiguity, I would say:

> Required. The Genome Reference Consortium identifier of the reference assembly. The currently valid values are `GRCh38`, `GRCh37`, `NCBI36`, with future reference genomes accepted.