Add a decay config that lets the EventDecayer decay charm baryons

This commit adds eventdecayer_charmbaryon.cfg, which switches on the decays of Xi_c0, Xi_c+ and Omega_c0 for the EventDecayer.

- base.cfg keeps these charm baryons stable, because Geant4 hands them to the external decayer one at a time.
- The EventDecayer decays them before transport and keeps the full decay chain in the MC truth.

Usage: -confKey 'DecayerPythia8.config[1]=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGHF/pythia8/decayer/force_hadronic_charmbaryon.cfg;EventDecayer.enable=true;EventDecayer.config[2]=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGHF/pythia8/decayer/eventdecayer_charmbaryon.cfg'

AliceO2Group/AliceO2#15995
https://its.cern.ch/jira/browse/O2-5903

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

MC/config/PWGHF/pythia8/decayer/eventdecayer_charmbaryon.cfg

@@ -0,0 +1,6 @@
+### Lets the EventDecayer (EventDecayer.enable=true) decay the charm baryons that base.cfg keeps stable.
+### Use it in a free EventDecayer.config slot next to the force_hadronic_charmbaryon*.cfg decay channels,
+### e.g. DecayerPythia8.config[1]=force_hadronic_charmbaryon.cfg;EventDecayer.enable=true;EventDecayer.config[2]=eventdecayer_charmbaryon.cfg
+4132:mayDecay = on     # Xi_c0
+4232:mayDecay = on     # Xi_c+
+4332:mayDecay = on     # Omega_c0