AthConfigFlags: add support for flag enums in fillFromArgs

This adds support for setting flags of type FlagEnum from the command line, e.g.:

athena --CA ... Beam.Type=BeamType.Cosmics

In passing also replace a few format calls with f-strings.

cc @tadej @wlampl

I am going to leave this as draft until !61413 (merged) from @will has landed in 23.0.