title: How to combine two components using allOf in openapi yaml specification?
body: <p>I try to generate java model classes from yaml specification using openapi-generator-maven-plugin:3.3.4.
  In one component I have allOf parameter trying to combine two different components by their refs. It goes like:</p>
  
  <p><code>Flight:
        allOf:
          - $ref: '#/components/schemas/FlightReference'
          - $ref: '#/components/schemas/FlightItem'
  </code></p>
  
  <p>What I got is Flight class which extends FlightReference, and FlightItem is not included in model at all. AFAIK in swagger-codegen-maven-plugin all properties from both classes was placed in class Flight, and current plugin introduces polymorphism instead, missing fields from second component. </p>
  
  <p>Is there a way to create class which combines all parameters from two other components as in swagger 2.0 plugin?</p>