[Enhancement] customCar property and car folder creation

[riv0tril]

Missing configuration

from the docs:

If set to a filename, the car, team and appearance will be used no matter what the user chose (Exception: overrideCarModelForCustomCar). This is useful for leagues and events, where we want consistent car appearance and the chosen car model for the corresponding driver/team. The custom car file has to be located in a “cars” folder next to the entrylist.json (also works for centralEntryListPath).
Leave blank (“”, =default) to let the user chose the car via car selection UI.