add new action in PoddAction to define overall rights to list
unpublished artifacts

this is necessary to trigger an authentication challenge, as there is no
other way to easily know whether they were allowed.

unpublished defaults to false for all users, but is turned on by default
for authenticated, so the action will not trigger for them
add alternative helper checkAuthentication method that does not require
object URIs, and can be used if the client knows that the action does
not require object URIs or they do not have any object URIs available.
add RDF listing code in draft form, not tested yet
add query parameters to list artifacts test