u/space_overseer,ipc: apply_stage_offset func

The apply_stage_offset function introduced here, exposed via IPC, allows an external application to apply translation and rotation offsets to the stage at runtime.

This is particularly useful for adjusting or fine tuning the one's play space, without having to interrupt the running XR session. Especially if the scene application doesn't have an adequate mechanism for re-centering.

Thanks to @RinLovesYou for the idea

PS: First PR here, so don't hold back on telling me everything that's wrong 😉