title: Create model classes with openapi generator which implements external interface
body: <p>I am using openapi-generator to generate java classes.</p>
  <p>I want the model classes to implement an external interface which has not been generated by openapi-generator.</p>
  <p>Is there something that can be defined within the model yaml or a property that can be passed to the openapi-generator-maven-plugin which allows for this behaviour?</p>
  <p>Example of required behaviour:</p>
  <pre><code>package com.example.model;
  
  /**
   * ExampleModel
   */
  @javax.annotation.Generated(value = &quot;org.openapitools.codegen.languages.SpringCodegen&quot;)
  public class ExampleModel implements com.example.CustomInterface {
    @JsonProperty(&quot;property1&quot;)
    private String property1;
  
    @JsonProperty(&quot;property2&quot;)
    private String property2;
  </code></pre>