Data provider: add option to tell a data server has no mirror data

[srosset81]

SPARQL queries can be quite long because we are also looking for data in the http://semapps.org/mirror graph.

But in some cases, for example with ActivityPods, we don't use this graph at all, so queries could be simplified.

We could add a noMirrorData option to the data servers to tell the data provider not to bother looking at this graph.

(Note: This would be a temporary fix, because when all resources will have their own named graph, we will remove this mirror graph)