Refactor PURL schema

[jamesaoverton]

I'd like to apply the suggestions from

https://json-schema.org/understanding-json-schema/structuring.html

to improve the JSON Schema for PURL configs:

https://github.com/OBOFoundry/purl.obolibrary.org/blob/master/tools/config.schema.json

The main improvement should be the use of "definitions" and "$ref" to avoid most repetition.

I'm putting the issue here for now since it's a bit of an experiment, but ultimately the successful result will be merged into the PURL repo.