title: Change return types for Spring openapi-generator-maven-plugin generated interfaces
body: <p>I've managed to generate the interfaces from a .yaml open-api descriptor file, but, as in the question title, I'd love to change the response type of those interfaces from ResponseEntity to my own types. Basically instead of the interface having this signature:</p>
  
  <pre><code> ResponseEntity&lt;Void&gt; clearCache();
  </code></pre>
  
  <p>for a method that is implemented basically this way:</p>
  
  <pre><code>public void clearCache(){ //do something}
  </code></pre>
  
  <p>I'd want the generated interface to be as simply as </p>
  
  <pre><code>void clearCache();
  </code></pre>
  
  <p>Same thing for my own defined classes, instead of <code>ResponseEntity&lt;MyBook&gt; getBook(String ISBN);</code> I want it to just use <code>MyBook</code> as return type, so it should look something like <code>MyBook getBook(String ISBN);</code>
  The current settings I'm using for the openapi-generator plugin are</p>
  
  <pre><code>        &lt;plugin&gt;
              &lt;groupId&gt;org.openapitools&lt;/groupId&gt;
              &lt;artifactId&gt;openapi-generator-maven-plugin&lt;/artifactId&gt;
              &lt;version&gt;4.3.0&lt;/version&gt;
              &lt;executions&gt;
                  &lt;execution&gt;
                      &lt;phase&gt;generate-sources&lt;/phase&gt;
                      &lt;goals&gt;
                          &lt;goal&gt;generate&lt;/goal&gt;
                      &lt;/goals&gt;
                      &lt;configuration&gt;
                          &lt;inputSpec&gt;my-own-service-be/src/main/resources/api-docs.yaml&lt;/inputSpec&gt;
                          &lt;generatorName&gt;spring&lt;/generatorName&gt;
                          &lt;additionalProperties&gt;
                               &lt;additionalProperty&gt;skipDefaultInterface=true&lt;/additionalProperty&gt;
                              &lt;additionalProperty&gt;interfaceOnly=true&lt;/additionalProperty&gt;
                          &lt;/additionalProperties&gt;
                          &lt;generateApis&gt;true&lt;/generateApis&gt;
                          &lt;apiPackage&gt;controller&lt;/apiPackage&gt;
                          &lt;supportingFilesToGenerate&gt;false&lt;/supportingFilesToGenerate&gt;
                          &lt;modelPackage&gt;dto&lt;/modelPackage&gt;
                          &lt;generateModelTests&gt;false&lt;/generateModelTests&gt;
                          &lt;generateApiTests&gt;false&lt;/generateApiTests&gt;
                      &lt;/configuration&gt;
                  &lt;/execution&gt;
              &lt;/executions&gt;
          &lt;/plugin&gt;
  </code></pre>