MDL-61692 core_privacy: add privacyfields to link_ methods

MDL-61692 core_privacy: add privacyfields to link_ methods

Some subsystems are like conduits, passing data along to plugins which,
in turn, send the data externally. Only the component linking to (using)
the subsystem will know what data it is sending along to the eventual
external system, so should be able to specify this at the time of
linking. The same goes for plugintypes.