GraphQL: Add support for rdfList

[Aklakan]

The @rdfList annotation should produce JSON arrays in JSON mode but nested objects (with the usual first/rest properties) in RON (RDF Object Notation) mode.

A goal is to make it possible to expose the intermediate list nodes. This means that at least internally the pattern ?rdf:rest* ?inter . ?iter rdf:first ?item needs to be constructed. Ideally, it would also be possible to provide a pattern and designate one variable as the intermediate one.

So what needs to be clarified is the interaction between @pattern and @rdfList.

Rough ideas:

• Have pattern point to the variable which carries the list, then use @rdfList on that variable.

{
   myList @pattern(of: "?s eg:authors ?l") @rdfList(from: "l", to: "item", via: "intermediate")
}

Generated pattern would be:

SELECT * {
  ?s eg:authors ?l .
  ?l rdf:rest* ?intermediate . ?intermediate rdf:first ?item .
}

• Point the pattern's target to the list items, and use @rdfList to declare the intermediate variable.

{
   myList @pattern(of: "?s eg:authors/rdf:rest* ?i . ?i rdf:first ?o", from: "s", to: "o")
          @rdfList(of: "?i")
}