21.6 save reduced truth particle container to EVNT for pileup minbias samples

This MR adds a new xAOD::TruthPileupParticleContainer output container which includes any final (no same-daughter) truth-level photons, electrons, or muons with pt > 8 GeV & eta < 4.5. This info will be added for the upcoming R22 minbias samples, and will be collected at digitization to provide pileup truth information. This container is only created if Generators/GeneratorFilters/share/common/AddPileupTruthParticles.py is added, and the output is only saved if evgenConfig.savePileupTruthParticles is set to true.

Tagging @jkatzy @cgutscho @jkretz @jchapman @mduehrss @zmarshal @ewelina for feedback on the placement and details of this functionality.

Jeff