Database Support: Virtuoso or generic SPARQL

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It would be interesting to be able to execute queries agaunst a SPARQL endpoint. This open metabase to a massive amount of data already existing out there, e.g: the whole [wikipedia](https://query.wikidata.org/).

If this is too difficult as it is not SQL, an alternative could be to support Virtuoso, that is a database that supports multiple technologies. In this case, the interesting point would be to run SQL against virtuoso but embedding SPARQL inside of the query with [SPASQL](https://medium.com/virtuoso-blog/spasql-about-8486deecba66).

The issue names no files, tests, or entry points. First clarify whether the goal is generic SPARQL endpoint support or Virtuoso with SPASQL, then review Metabase's database-driver architecture and existing query execution tests. Done requires an agreed scope and verified support for the selected integration.

Written by the indexing model from the issue text.