title: Firebase auth check for email already exists without using signInWithEmailAndPassword
body: <p>I would like to check that if a email already exists in firebase auth server then ask for password from user.</p>
  
  <p>There is a well known solution which is to use signInWithEmailAndPassword method and depend on the error message.  But in this case if the email does not exists it will create an account. I don't want to create an account I only want to know if the email already exists or not.</p>
  
  <p>Should I use a a cloud function for this purpose ? How can I do that ?</p>