title: Django Rest Framework regroup queryset by a category
body: <p>In the current project that i'm working on, i need to regroup (group) a queryset by category and put contents with same category in a list all provided together.
  I have the following model structure:</p>
  
  <pre><code>class Category(models.Model):
      title = models.CharField(max_length=255)
  
  class Item(models.Model):
     title = models.CharField(max_length=255)
     category = models.ForeignKey(to="Category", verbose_name=_('category'), related_name='items',
                                   on_delete=models.SET_NULL, null=True, blank=True)
  </code></pre>
  
  <p>I would like the output serialized result to be like:</p>
  
  <pre><code>    {
       "category_title_1":[
          {
            "id": 1,
            "title" : "something",
          },
          {
            "id": 2,
            "title": "something else",
          }
        ],
      "category_title_2": [
        {
         "id": 3,
         "title": "another string",
        }, 
        {
         "id": 4,
         "title": "and yet another title",
        }
      ]
    }  
  </code></pre>
  
  <p>I know i can always iterate over the queryset and group them manually, i'm wondering if there is a native efficient way to do this.</p>
  
  <p>Thanks</p>