Initial commit of MatchmakerExchange V1 API, as currently implemented in the wild.

This is an initial commit, designed to represent the V1 Matchmaker API as currently implemented in several systems. As such, there are a number of obvious improvements to be made and the coding conventions do not completely match GA4GH (e.g. enums are typically lowercase, since the API is currently implemented in JSON).

My understanding is that it is preferred to get a representative API into the repository first, and then iterate to improve.

This is a slightly-modified version of PR #137 by @cassiedoll.
This could be a union will null and an enum of the HPO ids, as they are known.