feat: use URNs as outputs

[cmdoret]

Context:

Uniform Resource Name (URNs) are URIs that use the urn scheme without any prefix.

Currently we hash the whole URI, and replace the fragment with the hash, retaining the original prefix. This poses 2 issues:

• we need to extract the prefix, which fails when urns are in the input
• sensitive data may be present in the prefix. This should be removed

Proposed solution

Using URNs (format: <urn:hash>) as output would provide a solution to both issues as the pseudonymized data would not have a namespace.

Objective: original domain is absent from pseudonymized URIs.

Requirements:

• Implement Hash for rdf types -> urn:
• (optional?) additional method to hash with optional domain

[cmdoret]

Possible formats:
urn:<hash>
urn:tripsu:<hash>
urn:uuid:<uuid>

[rmfranken]

I like urn:tripsu: but I want to see if there are best practices for crafting URN's. Maybe there is implicit meaning in the 'argument positions'. (like namespace:format in the case of urn:uuid or something). Don't want to accidentally introduce implicit meaning that we don't want.

[cmdoret]

This would be a good opportunity to implement Hash on the individual RDF types