add basic support for edge properties

[klinovp]

Not sure what the exact scope should be but at least it'd be good if construct { ?s ?p ?o} where { ?s ?p ?o } and select * {?s ?p ?o} worked on datasets with edge properties. It was reported in https://community.stardog.com/t/strange-construct-behavior-with-from-clause/4905/4 that it doesn't work.

For that pystardog should be able to parse and serialize both RDF-star and SPARQL results with edge properties.