This MR adds a new package
HGTD_G4_SD (package naming convention adopted from InnerDetector) which contains the code to actually create hits.
This code is meant to be the replacement for the old sensitive detector package which was removed in !35186 (merged). The difference being that the the geometry is included now in the InnerDetector rather than LAr as it was in 20.20. Also, the old code was producing both LAr and Si hits, while the replacement here will only produce Si hits.