Added TrackCollectionMergerAlg for disappearing tracks

This MR adds the TrackCollectionMergerAlg to the new CA config for disappearing tracks, which is (ab)used to compute some extra tracking information, as done in the legacy config here https://gitlab.cern.ch/atlas/athena/-/blob/master/InnerDetector/InDetExample/InDetRecExample/share/InDetRec_jobOptions.py#L1037

In principle, this could be extended to all standalone tracking collections, such as R3LargeD0, but for consistency with the legacy config, this is only added for the disappearing tracks.

This fixes the last q443 differences between CA and legacy configs, while maintaining the previous q442 agreement. (This in principle applies to both data and MC but the stat was probably too low in q442 to run into a case where the difference would show up.)