move enums to separate files

[BrapiCoordinatorSelby]

for the new Enum files, do they need to have null as an acceptable type? Or are $refs allowed to be null by default. That's the only thing that looks like it changed significantly

[BrapiCoordinatorSelby]

"type": [
    "null",
    "string"
]

[daveneti]

I am not sure, if allowed to be null (optional, not required) would not that be better to denote on the field that references the enum and not the enum itself? I will check the json schema spec.