[Java] Support inheritance (OAS3: discriminator/mapping element)

Description

Mapping element doesn't appear to be supported in io.swagger.codegen.v3:swagger-codegen-maven-plugin:3.0.3 plugin.

I have the following swagger (extract):

    Vehicle:
      type: object
      discriminator:
        propertyName: type
        mapping:
          car: '#/components/schemas/Car'
          truck: '#/components/schemas/Truck'
      properties:
        model:
          type: string
...

And the following class is generated:

@javax.annotation.Generated(value = "io.swagger.codegen.v3.generators.java.JavaClientCodegen", date = "2018-12-12T10:48:45.405+01:00[Europe/Paris]")@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "Discriminator{propertyName='type', mapping={car=#/components/schemas/Car, truck=#/components/schemas/Truck}}", visible = true )
@JsonSubTypes({
  @JsonSubTypes.Type(value = Car.class, name = "Car"),
  @JsonSubTypes.Type(value = Truck.class, name = "Truck"),
})
public class Vehicle {

Whereas, I would expect to find my swagger mappings in the name attribute, something like:

@JsonSubTypes.Type(value = Car.class, name = "car"),
@JsonSubTypes.Type(value = Truck.class, name = "truck"),

Specification link:
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#discriminatorObject

Sample project showing the issue: https://github.com/gonzalad/swagger-samples/tree/swagger-openapi3-inheritance/client

Swagger-codegen version

3.0.3

Swagger declaration file content or url

https://gist.github.com/gonzalad/1953b3c504240112bfc11241eb5afa38

Command line used for generation

            <plugin>
                <groupId>io.swagger.codegen.v3</groupId>
                <artifactId>swagger-codegen-maven-plugin</artifactId>
                <version>3.0.3</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/swagger.yml</inputSpec>
                            <language>java</language>
                            <configOptions>
                                <library>resttemplate</library>
                                <!--
                                <sourceFolder>src/gen/java/main</sourceFolder>
                                    -->
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

• run swaggercodegen on the linked swagger.yml file

Thanks,
Adrian

Reproduce the issue with the linked sample project's swagger.yml and the swagger-codegen-maven-plugin, then trace discriminator handling through JavaClientCodegen. Done means the generated Vehicle class uses the mapping keys "car" and "truck" as the JsonSubTypes name values and retains the discriminator property name "type".