title: Serialize nested properties as form-data with swagger-php
body: <p>This is how Encoding Object Example is done in OpenApi
  <a href="https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md" rel="nofollow noreferrer">https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md</a></p>
  
  <pre><code>requestBody:
    content:
      multipart/mixed:
        schema:
          type: object
          properties:
            id:
              # default is text/plain
              type: string
              format: uuid
            address:
              # default is application/json
              type: object
              properties: {}
            historyMetadata:
              # need to declare XML format!
              description: metadata in XML format
              type: object
              properties: {}
            profileImage:
              # default is application/octet-stream, need to declare an image type only!
              type: string
              format: binary
        encoding:
          historyMetadata:
            # require XML Content-Type in utf-8 encoding
            contentType: application/xml; charset=utf-8
          profileImage:
            # only accept png/jpeg
            contentType: image/png, image/jpeg
            headers:
              X-Rate-Limit-Limit:
                description: The number of allowed requests in the current period
                schema:
                  type: integer
  </code></pre>
  
  <p>I'm trying to achive the same thing but with swagger-php.
  What I don't know is how to pass <code>encodings</code> in <code>@OA\MediaType</code> to encode the <code>test</code> property as <code>multipart/form-data</code> because by default is encoded as <code>application/json</code></p>
  
  <p>EX:</p>
  
  <pre><code> * @OA\Post(
   *     path="/admin/test",
   *     summary="Create new Test",
   *     description="Will attempt to create a new Test",
   *     tags={"Admin Test"},
   *     @OA\RequestBody(
   *          @OA\MediaType(
   *              mediaType="multipart/form-data",
   *              encoding={}
   *              @OA\Schema(
   *                  type="object",
   *                  @OA\Property(
   *                      property="test",
   *                      type="object",
   *                      description="test"
   *                      ref="#/components/schemas/MyTestSchema"
   *                  )
   *              )
   *      )
  </code></pre>
  
  <p>They have some examples here:</p>
  
  <p><a href="https://github.com/zircote/swagger-php/tree/master/Examples" rel="nofollow noreferrer">https://github.com/zircote/swagger-php/tree/master/Examples</a></p>
  
  <p>but I didn't found any example regarding encoding</p>
  
  <p>Inside here the field is defined
  <a href="https://github.com/zircote/swagger-php/blob/master/src/Annotations/MediaType.php" rel="nofollow noreferrer">https://github.com/zircote/swagger-php/blob/master/src/Annotations/MediaType.php</a></p>
  
  <pre><code>   /**
       * A map between a property name and its encoding information.
       * The key, being the property name, must exist in the schema as a property.
       * The encoding object shall only apply to requestBody objects when the media type is multipart or application/x-www-form-urlencoded.
       */
      public $encoding = UNDEFINED;
  </code></pre>
  
  <p>I've tried <code>encoding={"recommended"={"contentType"="multipart/form-data"}}</code> but it's useless.</p>