[BUG] jaxrs-spec [server] oneOf, anyOf Polymorphism does not work

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Currently, openapi-generator doesn't support oneOf. It currently generates the following broken code, outputting UNKNOWN_BASE_TYPE rather than Pet for the example below.
The code is broken with or without the discriminator in the Spec.

Bug:

@Path("/pets")
@Api(description = "the pets API")
@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaJAXRSSpecServerCodegen", date = "2020-03-09T08:49:06.102+01:00[Europe/Copenhagen]")
public interface PetsApi {

    @POST
    @Consumes({ "application/json" })
    @ApiOperation(value = "", notes = "", tags={  })
    @ApiResponses(value = { 
        @ApiResponse(code = 200, message = "created", response = Void.class) })
    Response petsPost(@Valid UNKNOWN_BASE_TYPE UNKNOWN_BASE_TYPE);
}

Expected

Generator config:

           <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.2.3</version>

                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>

                        <configuration>
                            <inputSpec>${project.basedir}/src/main/webapp/spec/api.yaml</inputSpec>
                            <generatorName>jaxrs-spec</generatorName>
                            <modelNameSuffix>ApiModel</modelNameSuffix>

                            <configOptions>
                                <sourceFolder>src/gen/java/main</sourceFolder>
                                <dateLibrary>java8</dateLibrary>
                                <java8>true</java8>
                                <interfaceOnly>true</interfaceOnly>
                                <returnResponse>true</returnResponse>
                                <useSwaggerAnnotations>true</useSwaggerAnnotations>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

OpenAPI Spec:

openapi: "3.0.2"
info:
  version: 1.0.0
  title: Polymorphism
paths:
  /pets:
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - $ref: '#/components/schemas/Cat'
                - $ref: '#/components/schemas/Dog'
              discriminator:
                propertyName: pet_type
      responses:
        '200':
          description: created
components:
  schemas:
    # Parent
    Pet:
      type: object
      required:
        - pet_type      
      properties:
        pet_type:
          type: string
        eats:
          type: string
      discriminator:
        propertyName: pet_type
    # Child 1
    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          # all other properties specific to a `Dog`
          properties:
            bark:
              type: boolean
    # Child 2
    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          # all other properties specific to a `Cat`
          properties:
            climbs:
              type: boolean

openapi-generator version

4.2.3

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

1. create a pom.xml with openapi-generator-maven-plugin 4.2.3
2. create api.yaml file above
3. mvn clean compile
4. see broken code under target directory \target\generated-sources\openapi\src\gen\java\main\org\openapitools\api\PetsApi.java

Related issues/PRs

This bug is related to the Server side Java code (SpringBoot)
@bkabrda has already merged PR for client Jackson Client side: #5120

Suggest a fix

Try apply client side fix from #5120 to server side.

Reproduce the jaxrs-spec generation using the supplied api.yaml and Maven configuration, then inspect the generated target/generated-sources/openapi/src/gen/java/main/org/openapitools/api/PetsApi.java. Compare the server-side handling of oneOf with the client-side fix in PR #5120; done means generation produces a valid polymorphic request type instead of UNKNOWN_BASE_TYPE and the generated code compiles.