title: jsonSchema attribute conditionally required
body: <p>In jsonSchema you can indicate whether defined fields are mandatory or not using the &quot;required&quot; attribute:</p>
  <pre><code>{
      &quot;$schema&quot;: &quot;http://json-schema.org/draft-04/schema#&quot;,
      &quot;type&quot;: &quot;object&quot;,
      &quot;properties&quot;: {
          &quot;header&quot;: {
              &quot;type&quot;: &quot;object&quot;,
              &quot;properties&quot;: {
                  &quot;messageName&quot;: {
                      &quot;type&quot;: &quot;string&quot;
                  },
                  &quot;messageVersion&quot;: {
                      &quot;type&quot;: &quot;string&quot;
                  }
              },
              &quot;required&quot;: [
                  &quot;messageName&quot;,
                  &quot;messageVersion&quot;
              ]
          }
      },
      &quot;required&quot;: [
          &quot;header&quot;
      ]
  }
  </code></pre>
  <p>In certain cases, I would like the <em>messageVersion</em> field not to be mandatory. Is there any way to make the mandatory-ness of the this field conditional?</p>