When reading xAOD::TriggerMenuJson based metadata we look also for a collection of bunch group JSON objects.
If found they are loaded into memory and served to the user based on the value of the xAOD::BunchConfKey (which must also be written to the event store in every event)
These data are currently flagged as optional, this MR does not enable the writing of the data (needs follow up on [ATR-24394]) but will make the data available once POOL files with the appropriate content start to be produced in the future.