[ATR-20835] HLT truncation monitoring for R3

Follows on from !30003 (merged) (cc @sutt)

Moves the logic behind where to look for truncation from the HLTMonTool back into the TDT.

Additionally exposes the ConfSvc in this tool to python manipulation, as for R3 we should be pointing it to the xAODConfSvc rather than its default, the TrigConfSvc.

Prefers to check the flag in xAOD::TrigDecision (for R3 will aim to make this the canonical way offline). But for R2 input in Athena, it can also fall back on HLTResult_HLT

Uses new enum location from !30292 (merged)