Implement Version-diff method
As discussed in sshoc-marketplace#95 (closed) and the latest Curation Workflows document, we need a method by the backend that takes two versions as input and returns the difference between the two. The default input is the latest SUGGESTED/INGESTED version [N] and the latest ACCEPTED version [N-1] of an item, but in general it could be any two versions of one item (Approval scenario), or the latest (ACCEPTED) versions of two items (Deduplication scenario), or even a tentative new version of an item (as JSON-serialisation not yet stored in the system) vs. last ACCEPTED version of an item (Ingest update scenario). Output should be analogous to the JSON-serialisation of an item, but needs some additional structures to mark the differences.
(notify: @lbarbot, @klaus.illmayer, @mkozak, @ekalata)