title: How add a nested column to a dataframe in pyspark?
body: <p>I have a dataframe with a schema like:</p>
  
  <pre><code>root
   |-- field_a: string (nullable = true)
   |-- field_b: integer (nullable = true)
  </code></pre>
  
  <p>and I'd like to add a nested column to my dataframe, to have something like this:</p>
  
  <pre><code>root
   |-- field_a: string (nullable = true)
   |-- field_b: integer (nullable = true)
   |-- field_c: struct (nullable = true)
   |    |-- subfield_a: integer (nullable = true)
   |    |-- subfield_b: integer (nullable = true)
  </code></pre>
  
  <p>How can I achieve this in pyspark?</p>