KG APIs to return sort date

[jachro]

As a KG API user, I'd like the APIs to return the date used for sorting in the payload.

The idea.
We came up with the concept of a sort date to be returned in the payload so the API user can reason about the order of the items. In terms of the Dataset entity, the date would be either dateModified, dateCreated or datePublished in the exact form the search query uses internally.

Acceptance criteria:

• Find out a proper name for the date property in the payload so it reflects the meaning in the best possible way (sortDate ???)
• Make sure the Cross-Entity Search API conforms to the new concept
• Make sure the Project Datasets API conforms to the new concept