Support implict Offer policy type for DSP messages

[jimmarino]

Feature Request

This is related to #4117.

JsonObjectToPolicyTransformer should support letting a parent transform set an implicit type. For example, it should not be required to specify @type in the following DSP message (as is customary with Json-Ld):

{
  "@context":  "https://w3id.org/dspace/v0.8/context.json",
  "@type": "dspace:ContractOfferMessage",
  "dspace:providerPid": "urn:uuid:a343fcbf-99fc-4ce8-8e9b-148c97605aab",
  "dspace:dataset": "urn:uuid:3dd1add8-4d2d-569e-d634-8394a8836a88",
  "dspace:offer": {
    "@type": "odrl:Offer",
    "@id": "urn:uuid:d526561f-528e-4d5a-ae12-9a9dd9b7a518",
    "target": "urn:uuid:3dd1add8-4d2d-569e-d634-8394a8836a88"
  },
  "dspace:callbackAddress": "https://......"
}