Tweaks for GetUserArtifactAccessAsAdmin API

A couple of minor tweaks would make the GetUserArtifactAccessAsAdmin more useful by making it easier to identify which artefacts are specifically assigned to the user and which ones are in the list because they are widely shared

1. Add a parameter to provide option to exclude widely shared artefacts.
2. Add an attribute to identify those entries that are only there because it's a widely shared artefact (i.e. the user doesn't have explicit permissions)