Fast timing shashlik

Add the option to enable fast timing measurement in the Shashlik detector. The code is OFF by default and even when ON (enabled via --customise SLHCUpgradeSimulations/Configuration/combinedCustoms.cust_2023SHCalTime) it does not modify existing reco, just adds a new RecHit collection with fast timing information.

Details were presented in the Upgrade TP Performance Studies meeting:
https://indico.cern.ch/event/354694/contribution/7/material/slides/0.pdf