cubicweb-skos #5307471 [rdfio] Allow optional parameters in ``subjects`` & ``objects`` methods [in-progress]

Sometimes we want all objects for a predicate without knowing the subject. We should be able to do something like the following.

graph.objects(predicate_uri=<p_uri>)

Curently this is impossible, subject uri must be provided as in

graph.objects(entity_uri=<s_uri>, predicate_uri=<p_uri>)

An error is raised if subject_uri is not provided.

Same remarks apply to subjects method of a graph.