updated TTI customization: keep only the signal TrackingParticles; add c...

- updated customise_Digi_TTI in SLHCUpgradeSimulations/Configuration/python/combinedCustoms_TTI.py: keep only TrackingParticles from the main interaction
- minor update of l1EventContent_TTI and addition of a l1EventContent_TTI_forHLT, the latter may be used if we need to produce samples that may be used for HLT CPU studies
- added a cust_2023TTI_forHLT in combinedCustoms.py, which is the same as the default cust_2023TTI except that we call the "forHLT" event content