runHLT_standalone: migrate BFieldAutoConfig to flags

Set the default of HltEventLoopMgr.setMagFieldFromPtree to true and avoid setting it in runHLT_standalone. Also introduce corresponding Trigger.Online.BFieldAutoConfig flag to disable the magnet auto-configuration when needed.

TODO:

• Update https://twiki.cern.ch/twiki/bin/view/Atlas/TriggerOnlineMenuUploadRun3 once 23.0.31 is deployed at P1