Use event number from EventContext in G4Event (ATLASSIM-6635)

This is part of the work for ATLASSIM-6635. Previously Athena had ignored the G4Event eventID (just setting it to 1 everywhere), but this causes issues for Celeritas/AdePT, which use this eventID to follow multiple events in flight on the GPU. This merge request passes the EventContext from the top-level algorithms down to the method where the G4Event is created (ISF::InputConverter::ISF_to_G4Event(...)), so that it can be used to set the G4Event eventID uniquely for each event in the job.

Other clients of the affected interfaces also need to be updated, mostly they just ignore the new method arguments for now though.