SiDigitization changes for use by HGTD
Backporting changes from !42152 (merged) to allow !41919 (closed) to proceed.
Since the HGTD will use common ReadoutGeometry base classes, it should also use a common base for Digitization.
This update allows for HGTD Digitization to use SiChargedDiode/SiChargedDiodeCollection etc by making them use ReadoutGeometryBase classes rather than InDetReadoutGeometry.
Interface classes such as ISCT_Amp, ISCT_FrontEnd etc are added such that they can be used for HGTD as well. In a future MR the old interfaces will be moved out of SCT_Digitization, and the inheritance of the derived implementations there updated to use the new abstract interfaces added here.