Use relatedResource property from VC vocabulary instead of custom untp furtherInformation

The example on the VC data model spec suggests to associate additional resources using vcdm:relatedResource property. We should use this well-known mechanism instead of introducing our custom furtherInformation property and Link class.

In the example dpp credential we should replace this:

"furtherInformation": [
        {
          "type": [
            "Link"
          ],
          "id": "https://files.example-certifier.com/1234567.json",
          "name": "GBA rule book conformity certificate",
          "digestMultibase": "abc123-example-digest-invalid",
          "mediaType": "application/ld+json",
          "linkType": "https://test.uncefact.org/vocabulary/linkTypes/dcc"
        },

with this:

"relatedResource": [
        {
          "type": [
            "untp:DigitalConformityCredential"
          ],
          "id": "https://files.example-certifier.com/1234567.json",
          "name": "GBA rule book conformity certificate",
          "digestMultibase": "abc123-example-digest-invalid",
          "mediaType": "application/ld+json",
        },