title: How to get all users having custom claim as admin/moderator using firebase admin?
body: <p>I want to send welcome message to all admins of my user database. Is there any method in firebase so that I can get list of all users with their custom claim as admin set to be true.</p>
  
  <pre><code>admin.auth().getUsers([
      { uid: 'uid1' },
      { email: 'user2@example.com' },
      { phoneNumber: '+15555550003' },
      { providerId: 'google.com', providerUid: 'google_uid4' },
    ])
  </code></pre>
  
  <p>Would above method applicable? If yes, then what parameter should I pass?</p>