Implement external EnumTypes

[OkieOth]

{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "ExternalExampleEnum",
"description": "example schema that contains only one enum",
"version": "0.0.1",
"type": "string",
"enum": ["A", "B", "C"]
}

[OkieOth]

Done