A service providing a method to retrieve a UnionGraph containing
the ContentGraph and additional graphs.
Over the Configuration Admin Service additional graphs can be added
permanently. With the methods addTemporaryAdditionGraph and
removeTemporaryAdditionGraph graphs can be added temporarily.
Temporarily means that after restarting the org.apache.clerezza.platform.graphprovider.content
bundle, these graphs will no longer be returned in the UnionGraph.