Report uses of anonymous classes in outgoing Remoting packets

JENKINS-49795 issues warnings about both reads and writes of anonymous (and similar) classes for XStream, because XStream2.BlacklistedTypesConverter calls ClassFilter.DEFAULT in both directions. But currently these warnings are issued only for incoming Remoting packets—either slave → master callables, or return types of master → slave callables. The much more common case of anonymous (etc.) classes being used for master → slave callables is not detected.