[Kotlin] [Client] Add support for x-kotlin-implements and x-kotlin-implements-fields

[Mattias-Sehlstedt]

Uses the functionality introduced with #21950 to support interface-inheritance definitions with vendor extensions for client generation as well. This is for example useful when the schemas in a oneOf does not refer to some shared schema that carries the discriminator.

Specification example:

openapi: 3.0.1
info:
  title: some
  version: 1.0.0
servers:
  - url: http://localhost:8080
    description: Local server
paths:
  "/one-of-with-without-all-of-inheritance":
    get:
      operationId: WithoutAllOfEndpoint
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                oneOf:
                  - "$ref": "#/components/schemas/Child1"
                  - "$ref": "#/components/schemas/Child2"
                discriminator:
                  propertyName: jobType
                  mapping:
                    Child1: "#/components/schemas/Child1"
                    Child2: "#/components/schemas/Child2"
components:
  schemas:
    Child1:
      type: object
      properties:
        jobType:
          type: string
        property1:
          type: string
    Child2:
      type: object
      properties:
        jobType:
          type: string
        property2:
          type: string

Note that this does not solve the issue by itself, since the generated interface will look like

@JsonIgnoreProperties(
  value = ["jobType"], // ignore manually set jobType, it will be automatically generated by Jackson during serialization
  allowSetters = true // allows the jobType to be set during deserialization
)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "jobType", visible = true)
@JsonSubTypes(
    JsonSubTypes.Type(value = AllOfChild1::class, name = "AllOfChild1"),
    JsonSubTypes.Type(value = AllOfChild2::class, name = "AllOfChild2")
)

interface WithAllOfEndpoint200Response {

    @get:JsonProperty("jobType")
    val jobType: kotlin.String?
    @get:JsonProperty("property1")
    val property1: kotlin.String?
    @get:JsonProperty("property2")
    val property2: kotlin.String?

}

rather than the more correct

@JsonIgnoreProperties(
  value = ["jobType"], // ignore manually set jobType, it will be automatically generated by Jackson during serialization
  allowSetters = true // allows the jobType to be set during deserialization
)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "jobType", visible = true)
@JsonSubTypes(
    JsonSubTypes.Type(value = AllOfChild1::class, name = "AllOfChild1"),
    JsonSubTypes.Type(value = AllOfChild2::class, name = "AllOfChild2")
)

interface WithAllOfEndpoint200Response {

    @get:JsonProperty("jobType")
    val jobType: kotlin.String?

}

(what has been corrected is that fields that are not the discriminator has been removed).

This is tied to a separate issue that I plan to adjust for in a separate PR. It has to do with how a discriminator is interpreted and generated in the data_class.mustache and the

{{#discriminator}}{{#vars}}{{#required}}
{{>interface_req_var}}{{/required}}{{^required}}
{{>interface_opt_var}}{{/required}}{{/vars}}{{/discriminator}}

definition.

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