Add ability to build
AthGeneration writing out legacy
The motivation for this is to avoid building legacy branches to have working EVNT merge that writes out
McEventCollection_p5. This is needed for MC16 and MC20 sample production with EVNT made in master-based releases.
While probably some old 22.6 release is enough, this took me very little time to implement and test and now have it ready even if we do not decide to merge. Keeping draft for now to allow discussion.