GH-4998: Add JoinQuery for easy and efficient creation of joins in rdf4j spring DAOs

[fkleedorfer]

GitHub issue resolved: #4998

Briefly describe the changes proposed in this PR:

The RelationMapBuilder generates join queries on the fly, and might be in use by some users. Instead of changing it, ths PR introduces JoinQuery and friends, which offer a better solution to the same problem - they produce reusable queries transparently, allow for specifying constraints on source and target entity, and provide intuitive options for the type of join (inner/left outer/right outer).

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[fkleedorfer]

Sorry, had to convert this to draft. I am stumbling over the next iteration of this concept, it might take a while to update this PR.