Support OpenAPI `oneOf`-property

[Chrimle]

[Chrimle]

This will require a new .mustache template, as the field marked as oneOf is generated as a separate class.

This is generated by this template from openapi-generator-maven-plugin: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache