feat: add RDFGraph to API

[rokroskar]

Adds an RDFGraph object to the API

Usage:

from renku.ui.api import RDFGraph

g = RDFGraph()
# get a list of contributors to the project
list(g.subjects(object=URIRef("http://schema.org/Person")))

[Panaetius]

Should probably add it to https://github.com/SwissDataScienceCenter/renku-python/blob/develop/docs/reference/api.rst as well.

Otherwise looks good, thanks!

[rokroskar]

Thanks @Panaetius - done