title: Can Avro schema have unknown / dynamic field names?
body: <p>Consider the following json:</p>
  <pre class="lang-json prettyprint-override"><code>{
    &quot;id&quot;: &quot;5ffhj4ksWRyt&quot;,
    &quot;name&quot;: &quot;some name&quot;,
    &quot;custom_fields&quot;: [
      &quot;randomFieldName1&quot;: &quot;a&quot;,
      &quot;randomFieldName2&quot;: &quot;b&quot;
    ]
  </code></pre>
  <p><code>id</code> and <code>name</code> fields are always known, but <code>custom_fields</code> names are dynamic and can have any key name. Is it possible to write a schema that will take that into account ?</p>