Egamma config smarter flags add egamma track slimmer

Following discussion with @tbold make flags a bit smarted add egammaTrackSlimmer alg (in reality we call what it does slimming these days).

This comes after !47485 (merged) and fixes some issues like

File "/tmp/christos/Athena-git/build/x86_64-centos7-gcc8-opt/python/egammaConfig/egammaBuilderConfig.py", line 36, in EGammaReconstructionCfg
    flags.Egamma.doTruthAssociation = False
  File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc8-opt/2021-10-21T2101/Athena/22.0.46/InstallArea/x86_64-centos7-gcc8-opt/python/AthenaConfiguration/AthConfigFlags.py", line 97, in __setattr__
    return self._flags._set( merged, value )
  File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc8-opt/2021-10-21T2101/Athena/22.0.46/InstallArea/x86_64-centos7-gcc8-opt/python/AthenaConfiguration/AthConfigFlags.py", line 231, in _set
    raise RuntimeError("Attempt to set a flag of an already-locked container")

see [http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/MASTERWebArea/ardoc_web_areamaster64BC7G8AthenaDbg/ARDOC_TestLog_2021-10-22T2101/Reconstruction_RecJobTransforms___RecJobTransformsConf__RecJobTransformsTest__m.html]

ping @jdevivi