title: How to flatten polymorphic lists with Moshi deserialization
body: <p>I'm trying to flatten the following Json</p>
  
  <pre><code>[
    {
      "type": 1,
      "field": ""
    },
    {
      "type": 2,
      "field": "",
      "other_field": ""
    },
    {
      "type": 3,
      "field": "",
      "list_field": [
        {
          "type": 4,
          "some_field": ""
        },
        {
          "type": 4,
          "some_field": ""
        }
      ]
    }
  ]
  </code></pre>
  
  <p>what I'd like to obtain is the following structure:</p>
  
  <pre><code>[
    {
      "type": 1,
      "field": ""
    },
    {
      "type": 2,
      "field": "",
      "other_field": ""
    },
    {
      "type": 3,
      "field": ""
    },
    {
      "type": 4,
      "some_field": ""
    },
    {
      "type": 4,
      "some_field": ""
    }
  ]
  </code></pre>
  
  <p>Since all the items inherit from the same parent class I tried both PolymorphicJsonAdapterFactory and a custom JsonAdapter.Factory but without success.</p>
  
  <p>Is there a way to do that in Moshi?</p>