Add STGC xAOD PRD

Following on from !66087 (merged), !66022 (merged) and !66218 (merged).

As with those, this is adding new functionality for HL-LHC preparations and this does not run in production.

This contains !66218 (merged) and so I'll make it draft until this is merged, to save this code getting reviewed twice.

I should have mentioned the ticket: ATLASRECTS-7674