dynamically detect tracker plugins and cache them instead of using config

refs #6065 

FYI: A plugin now has to listen to any `Tracker` or `Request.initAuthenticationObject` event or it has to define dimensions in order to be detected as a tracker plugin. Before it was possible to add it manually to the PluginsTracker list in config which is no longer the case. 

FYI: We can rename the update script later to another version but 2.6.0 would be ok. It is not needed to be removed from the config directly as it will be ignored anyway.