title: Use LinkedHashMap instead of HashMap for Swagger dictionary type (Java codegen)?
body: <p>I'm using openapi 3.0.2, and the codegen plugin:</p>
  
  <pre><code>&lt;groupId&gt;io.swagger.codegen.v3&lt;/groupId&gt;
  &lt;artifactId&gt;swagger-codegen-maven-plugin&lt;/artifactId&gt;
  &lt;version&gt;3.0.14&lt;/version&gt;
  </code></pre>
  
  <p>I'm using the swagger dictionary/hashmap type described here:</p>
  
  <p><a href="https://swagger.io/docs/specification/data-models/dictionaries/" rel="nofollow noreferrer">https://swagger.io/docs/specification/data-models/dictionaries/</a></p>
  
  <pre><code>openapi: "3.0.2"
  ...
  Labels:
      type: object
      additionalProperties:
          type: string
          minLength: 1
      description:  A description.
  </code></pre>
  
  <p>When I generate Java code for this, it is modelled as a class which extends <code>HashMap</code>:</p>
  
  <pre><code>@ApiModel(description = "A description.")
  @Validated
  @javax.annotation.Generated(value = "io.swagger.codegen.v3.generators.java.SpringCodegen", date = "2020-02-26T12:17:36.248Z[Europe/London]")
  public class Labels extends HashMap&lt;String, String&gt;  {
  ...
  }
  </code></pre>
  
  <p>Is there any way to instruct swagger to use a <code>LinkedHashMap</code> instead of a <code>HashMap</code>? (without having to exclude this class from codegen and manually modify it). </p>
  
  <p>I would like to control the ordering of the entries in this dictionary when returning it to clients.</p>