Add BDT tau algorithm flag in configuration

Following https://its.cern.ch/jira/browse/ATR-28687, this implements subtask 1 of the suggestion by @thompson .

Trigger/TriggerCommon/TriggerJobOpts/python/TriggerConfigFlags.py

@@ -222,6 +222,9 @@ def createTriggerFlags(doTriggerRecoFlags):
                   prevFlags.Trigger.L1.doCalo and prevFlags.Trigger.enableL1CaloPhase1,
                   help='enable eFEX ByteStream conversion/simulation')
 
+    flags.addFlag('Trigger.L1.doeFexBDTTau', True,
+                  help='use BDT tau algorithm as the active one for eFEX')
+
     flags.addFlag('Trigger.L1.dojFex', lambda prevFlags:
                   prevFlags.Trigger.L1.doCalo and prevFlags.Trigger.enableL1CaloPhase1,
                   help='enable jFEX ByteStream conversion/simulation')