[REQ][Java] support anyOf Enum

Created by: lwlee2608

3GPP R15 has defined a set of standards in the form of OpenAPI specification files.

The files are often lengthy and consist of oneOf, anyOf, allOf keywords.

The following definition that combines anyOf with enum has been consistently poping up and it will break the openapi-generator

    RequestTrigger:
      anyOf:
      - type: string
        enum:
          - LOC_CH
          - PRA_CH
          - SERV_AREA_CH
          - RFSP_CH
          - ALLOWED_NSSAI_CH
          - UE_AMBR_CH

The issue has been raised in #798 The solution was just workaround the spec files.

However it would be nice if we can instead add an option to the generator to simply ignore the anyOf. Ideally we want the generator to work without any alteration to the spec files.