Expose more of Oxigraph as rdflib plugins

[Tpt]

rdflib allows to register plugins for I/O and SPARQL processing. It might be nice to expose oxigraph features using it to allow rdflib users to opt-in to use some oxigraph implementations instead of the rdflib ones.

[nikokaoja]

@Tpt , how can we collaborate on this?

[Tpt]

RDF formats parsers and serializers are now exposed, allowing also nice optimizations when used alongside the Oxigraph store