Common: support references

[chuwy]

https://spacetelescope.github.io/understanding-json-schema/structuring.html

This was raised multiple times, mostly in order to reduce boilerplate code, but we never actually considered it closely enough. Now @dilyand found another good application for it in data-modeling with graph-oriented DBs.

Wondering if this should classic JSON Schema $ref or we can use iglu:-protocol to link schemas.

[alexanderdean]

I thought about this a couple of years ago. I think we would have to use $ref with iglu: protocol, and the JSON Schema library (?) would need to natively understand iglu: as a protocol...