Use case: analysis of edits for revision scoring
wbentitydiff should support:
* the same parameters for controlling diffing as index.php does with diff and oldid.
* getting the diff between an entity's current revision and the previous revision, by supplying just the entity ID.
* diffing any revisions of any entities of the same type
The diff should be represented as a json structure, just as we do in DiffChange.
Implementation notes:
* Follow the exampel of the wbgetclaims module (see the GetClaims class).
* Get the desired EntityRevisions using an EntityLoadingHelper
* Create a EntityDiff using the appropriate EntityDiffer (TBD: provide a factory for this)
* Convert the EntityDiff to a JSON-style array structure (TBD: provide an DiffSerializer for this)
* Return the resulting array structure via the ApiResult (possibly using ResultBuilder)