Geant4 benchmark integration

[sethrj]

Based on discussions today with @stognini @amandalund @whokion and @pcanal

• Split SimpleOffload into ManagerIntegration/ActionIntegration classes: the first (preferred, requires G4 11.1) uses the G4TrackingManager and doesn't need begin/end events; the second is for older versions that uses a tracking action.
• Updated Integration classes (only one can be used at a time) will own the global and thread-local objects that previously were boilerplate; and it will provide safe access to the setup options during setup.
• Eventually need to add region/volume-based field description (map to AlongStepId)
• Individual integration: see https://github.com/celeritas-project/gpu-delta-review-2025/issues (private repo for now)