[relationshipEvaluation] Enhance returnDiffOnly property to have structured "addition" and "deletion" results

[leecalcote]

schemas/schemas/constructs/openapi/relationshipEvaluation.yml

Line 42 in 9878dcc

returnDiffOnly:

Without this refined structure, clients of this API will be left to figure out what has been added and what has been removed, since the definition of "addition" and "deletion" is not accounted for in the schema.