title: BigQuery: Create column of JSON datatype
body: <p>I am trying to load json with the following schema into BigQuery:</p>
  
  <pre><code>{
  key_a:value_a,
  key_b:{
     key_c:value_c,
     key_d:value_d
    }
  key_e:{
     key_f:value_f,
     key_g:value_g
    }
  }
  </code></pre>
  
  <p>The keys under key_e are dynamic, ie in one response key_e will contain key_f and key_g and for another response it will instead contain key_h and key_i.  New keys can be created at any time so I cannot create a record with nullable fields for all possible keys.  </p>
  
  <p>Instead I want to create a column with JSON datatype that can then be  queried using the JSON_EXTRACT() function.  I have tried loading key_e as a column with STRING datatype but value_e is analysed as JSON and so fails.</p>
  
  <p>How can I load a section of JSON into a single BigQuery column when there is no JSON datatype?</p>