Update a User's attributes.
Note: In order to add a new email address or phone number to an existing User object, pass the new email address or phone number into the respective /send endpoint for the authentication method of your choice. If you specify the existing User's user_id while calling the /send endpoint, the new, unverified email address or phone number will be added to the existing User object. If the user successfully authenticates within 5 minutes of the /send request, the new email address or phone number will be marked as verified and remain permanently on the existing Stytch User. Otherwise, it will be removed from the User object, and any subsequent login requests using that phone number will create a new User. We require this process to guard against an account takeover vulnerability.