Kotlin inheritance is broken when using Enums

swagger-core currently generates enums inlined. I've opened a corresponding bug to be able to pull these classes out into their own enum referenced object which seems to be supported in OAS3, https://github.com/swagger-api/swagger-core/issues/3124.

Either way, the kotlin-client generator is currently generating non-compilable Kotlin code.

For example, this is what's derived from swagger-core:

{
  "openapi" : "3.0.1",
  "info" : {
    "title" : "Test",
    "contact" : {
      "email" : "support@test.com"
    },
    "version" : "1.0"
  },
  "paths" : {},
  "components" : {
    "schemas" : {
      "Parent" : {
        "type" : "object",
        "properties" : {
          "enumeration" : {
            "type" : "string",
            "enum" : [ "TEST" ]
          }
        }
      },
      "Child" : {
        "type" : "object",
        "properties" : {},
        "allOf" : [ {
          "$ref" : "#/components/schemas/Parent"
        } ]
      }
    }
  }
}

When generating the Kotlin model classes with swagger-codegen, this generates exploded classes because Kotlin data classes cannot be inherited. The issue is the Child's reference to the Enumeration class which does not exist.

Parent.kt

/**
 * 
 * @param enumeration 
 */
data class Parent (

    val enumeration: Parent.Enumeration? = null
) {
    /**
    * 
    * Values: TEST
    */
    enum class Enumeration(val value: kotlin.String){
        TEST("TEST");
    }
}

Child.kt

/**
 * 
 */
data class Child (

    val enumeration: Child.Enumeration? = null
) {
}

Start with the kotlin-client generator and compare the generated Parent.kt and Child.kt models shown in the issue. Reproduce the schema, trace how inherited properties and inline enums are rendered, and verify that the generated Kotlin classes compile without an unresolved Child.Enumeration reference.