WIP: Add a PixelSensorGmxSD and related configuration, to support sensitive GeoModelXML Pixel detector elements with ITk

This MR attempts to remove the placeholder warning and allow at least at a primitive level to add sensitive detector elements for ITk Pixels modeled using GeoModelXML.

Intended for now primarily as an additional validation option for the ongoing simulation development, to eventually allow printouts of the Hits being recorded in G4.

CC @tlari @nstyles for expert feedback before considering un-WIPing.