allow simulation to be run on ESD file

This is a re-attempt at the work attempted by !64214 (closed) ... main difference here is that instead of using the INFILE trigger config option, which then requires use of the xAODConfigSvc, I make use of the FILE trigger config option, since the json files are generated at configure-time. @dreikher can you comment on whether it is really necessary or not to satisfy the https://its.cern.ch/jira/browse/ATR-26325 ticket for us to use the INFILE configuration, or do you agree we are fine to use the FILE option?

I tested this code by running:

python -m L1CaloFEXSim.L1CaloFEXSimCfg -i path/to/input.AOD.pool.root -e -n 5

Using the test AOD.pool.root file David provided for me.