title: Authorize domain calls
body: <p>I have a JS API that uses firebase internally for DB operations, API will be used on different website domains.
  I need to restrict DB operations calls from authorized domains only.</p>
  
  <p>I cannot restrict by UID as non signed user also need to access website.
  I believe i need to figure out some way by anonymous user or else any better way, any suggestions ?</p>