Add version info to JSON Schema, and implement a procedure to ensure in sync with reference implementation

[cmungall]

Currently the JSON-Schema is purely derived. The procedure is to run SchemaGeneratorTest in the reference implementation. The Makefile in this repo copies this across. This is potentially confusing, things can get out of sync.

• The JSON-Schema should be tagged with version info, and this should sync'd with the reference implementation release on mvn central
• There should be a proper maven assembly target to generate the schema in the target/directory. Should this be part of the release?
• The format and reference repos should be better synced.
• The overall process should be better documented. We have minimal docs here: https://github.com/phenopackets/phenopacket-format/wiki/JSON-Schema

In many ways a merger of the two repos might make some of this easier