COMUNICA: support custom comunica configurations

[jacoscaz]

As of v7.1.0, quadstore does not support any Comunica configuration other than quadstore-comunica, which optimizes for bundle size and dependency count to deliver the smallest possible Comunica configuration that still manages to evaluate queries.

comunica-quadstore does not include anything else but the subset of actors that are required to get from a SPARQL non-federated query to its RDF/JS results, leaving out parsers, serializers, federation, GraphQL and a bunch of other features supported by Comunica.

We should probably allow developers to pass custom engine factory functions to quadstore's constructor.

[jacoscaz]

Released with quadstore@8.0.0-alpha.3 (watch out for the new required constructor option!).

[markmelville]

I'm experimenting with quadstore as a server that fronts a small amount of data, in a distributed linked data network. As such, I'd like to be able to execute federated queries.

This issue and its resolution seem to indicate that, although quadstore-comunica has chosen not to implement it, I should be able to provide a custom function. I can't find any examples, docs, or code that is helping me to see how to do this. Can anyone provide some pointers? Thanks in advance.