Import HGTD GeoModel from 20.20

This MR brings in the HGTD GeoModel from 20.20 as a first step in getting the HGTD up and running in 21.9, creating a new package structure for HGTD according to ATLSWUPGR-66.

A couple of slides were shown recently in the first of a series of meetings covering 21.9 HGTD integration.

HighGranularityTimingDetector/HGTD_Conditions/README.md

+For now this dir holds empty folders representing the future HGTD packages in 21.9
\ No newline at end of file