Approved changes to Matchmaker API for v2, from Google doc

This is a collection of changes to the API that were proposed and approved in the Google doc for the draft v2 API: https://docs.google.com/document/d/1sDp_IYenhk3kdCCEEcFL_GAPVJqpB0TDPNUC9pZ4xo8

In order to be incorporated, these changes had to be approved by representatives from 3 different matchmaker groups, with no unresolved objections.

Here is a summary of those changes:
* Changed inheritance mode to use HPO terms
* Changed variant type to use Sequence Ontology
* Remove /mmapi section of the endpoints, since this API path can be negotiated as part of the <base_remote_url>
* Made “observed” field for features boolean-compatible
* Added “id” to response, to more closely match request schema
* Moved “assembly” to optional top-level field