Assign AllDatabasesViewer role at Eventhouse level.

[nicola_greco]

At the moment it is possible to assign AllDatabasesAdmin and AllDatabasesViewer to Eventhouse inheriting the Admin or Viewer permission of the workspace, implying that Admin or Viewer role is assigned to all Eventhouse artifacts in the workspace.

It would be great if AllDatabasesAdmin and AllDatabasesViewer can be assigned to individual Eventhouse artifacts in the workspace.