Add support for presenting multiple Verifiable Presentation in `vp_token`

[babisRoutis]

According to OpenId4VP a vp_token can represent one or more verifiable presentations.

In particular

in a case of a single verifiable presentation, vp_token is either

• a String (for mso_mdoc, SD-JWT-VC or W3C VC DM JWT), or
• a Json Object (for W3C VC DM JSON-LD)

in case of multiple verifiable presentations vp_token is a JSON array where each element is either a String or a JSON Object as described above.

Currently, library support single verifiable presentation using a String.

Support for the other cases should be provided