[Secure]Link's `linkURL` is not an `@id` type

Created by: absoludity

Issue Description

I'm not sure if this is an issue, but while looking at Tier 3 testing for credentials, I've noticed that from the current model, within the DigitalProductPassport's list of claims, each claim has a conformityEvidence property which from the property definition, should be:

A URI pointing to the evidence supporting the claim. SHOULD be a URL to a UNTP Digital Conformity Credential (DCC)

The type of the property is untp.SecureLink, which is a class that has multiple properties, one of which is the linkURL.

Now in a graph, the linkURL is just another property - it's not a node (it doesn't have an @id type), and so, no connection will be drawn between this property and the DCC which has the same URL as it's @id. This may not be an issue, because the DCC will itself have a link from it's product to the product of the DPP, so there will be a link there in that case, but I'm not sure if we also want the semantic link from the evidence to the DCC.

In other cases where the conformityEvidence is a simple PDF for example (also allowed), the fact that there is no node means that, for example, if multiple claims reference the same conformityEvidence value, that value will just remain a leaf value of each claim, rather than the graph linking all those claims to the same conformity evidence node.

If I get a chance, I'll update this ticket with a couple of jsonld playground examples that show the difference in the simplified graph.