Add support for .json options to Gaudi.exe

This is a follow up on !1264 (merged) making possible workflows like:

gaudirun.py my_complicated_options.py -n -o opts.json
gdb --args Gaudi.exe opts.json

(which is faster than gdb --args python $(which gaudirun.py) opts.json)

Note that this can be used in LHCb modern applications only in conjunction with lhcb/LHCb!3825 (merged)