ATR-31707: automatically figure out where the BTag decorations are on

Closes ATR-31707.

Tested locally with test_trigAna_RDOtoRun3DQ_v1Dev_build.py. Everything should work.

Tagging @giagu and @tamartin for trigger.

Tagging @dbaronmo @dguest @svanstro @robouque for FTAG.