[Feat]: Add endpoint to query for all indexed predicates.

[matglas]

Describe the solution you'd like:

It would be great if there is a query endpoint like subject to query for all known predicates. You could even extend that to dsse or subjects that are related to that predicate. Maybe like the first, last, etc. to get a number of subjects.

User value:

It would allow a implementation for a UI to easily give direction on discovery into the different types of data that are in a storage.

Expected behavior:

query {
  predicateTypes(first: 10, where: { predicateHasPrefix: "https://witness.dev/example-predicate" }) {
  }
}

Proposed solution:

Extend the implementation.

Testing changes required:

• Test with a few different predicates and returning those thru query.
• Test with one.
• Test with different has prefix.
• Test with last X and having more items then X.

Documentation changes required:

Extend the documentation for GraphQL. Probably auto generated.