[JAVA] [SPRING] Extensible enum for spring generator using oneOf. Fix #7272

[jpfinne]

Allow enumerations to be extensible. It greatly reduce the risk of breaking changes when new enums are added at a later date.

Similar to issue described in #7272 solution 3 using oneOf.

The PR adds the jackson annotation to the base interface so that deserialization and serialization work.

@JsonDeserialize(using=Country.EnumDeserializer.class)
public interface Country {
    @Valid
    @JsonValue
    String getValue();
    ...

Two versions are supported by the generated deserializer:

1. oneOf enum/string
2. oneOf enum/enum

for 1) a custom inner class is created with bean validation supported

class CountryString implements Country {
   @Override
   public String getValue() {
    }
}

Example contract

  Country:
     description:  version 1) enum or string    
      oneOf:
        - $ref: '#/components/schemas/SubsetCountry'
        - type: string
          description: other coutries
          pattern: ^[A-Z]{2}$
    SubsetCountry:
      type: string
      enum:
        - BE
        - LU
        - NL
    countryOrOther:
      description: version 2) enum or enum
      oneOf:
        - $ref: '#/components/schemas/SubsetCountry'
        - $ref: '#/components/schemas/OtherCountryEnum'
    OtherCountryEnum:
      type: string
      enum:
        - IT
        - DE
        - FR

It works for Spring with the new configOptions useOneOfExtensibleEnums=true. It requires jackson=true and useOneOfInterfaces=true (both true by default for the Spring generator)

The current PR is only for Spring. Other java generators supporting Jackson can implement this feature by importing oneof_interface.mustache. I can perform it when this PR is approved.

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