Consider adding source version numbers

[edeutsch]

From Chris Bizon https://github.com/NCATSTranslator/Feedback/issues/1322

When we have a PK, we can't currently tell which version of the graph was used to create the result. We should include some kind of graph provenance in the TRAPI somewhere. Assigning to @edeutsch to consider in TRAPI.

[edeutsch]

After some discussion at the hackathon, it was decided that adding version tags to every source object would be unwieldy. Instead we you should add a version tag or a URL to the complete dump of version metadata available at the Retriever API.