title: composite key in google cloud datastore
body: <p>In GCP Datastore &amp; I've a kind with 3 properties.</p>
  
  <ol>
  <li>key (auto-generated)</li>
  <li>externalId</li>
  <li>externalName</li>
  </ol>
  
  <p>I can't allow duplicate externalName + externalId combination. Is it possible to maintain this uniqueness?</p>