Add generic Ontop backend

[jesper-friis]

Ontop is a knowledge graph system that exposes a range or major relational database systems as a triplestore. See https://ontop-vkg.org/

Adding a generic Ontop backend would allow us to utilise relational database as the storage backend for Tripper. A use case for this could e.g. be when you lack access to a proper triplestore, but have access to a relational database.

The question is if this is useful backend to include...