Support ref and callbackUrl in presentation-request endpoint

[genaris]

In order to make it possible to create backend for the Generic Verifier DTS for UnicID, Service Agent must add support for asynchronous callbacks in the Presentation Request flows.

This will mainly consist on adding ref and callbackUrl to invitation/presentation-request endpoint so Service Agent client can keep track of the results of the presentation request in a simple manner (i.e. without the need to conform to the whole Service Agent API, which could be cumbersome for this simple purpose).