Cleanup SimConfigFlags + add enums
Cleanup SimConfigFlags and add enums for flags that are suitable for that.
SimulationFlavour is a bit special as it has isQuasiStable and isMT helpers. We may also want to cleanup old LongLived names (but that will come fully in a follow-up MR).
Note that TruthStrategy seems to be a bit inconsistent and for now there are some commented-lines to warn about missing configuration which will be cleaned up in the future.