Don't force sync attributes on user registration

Problem/Motivation

When a new user is provisioned, the account name and email are synced with the SAML attributes. However, our IdP does NOT return a mail attribute, so an exception is thrown during the sync:

// SimplesamlphpDrupalAuth.php
public function getAttribute($attribute) {  <-- $attribute is blank
    $attributes = $this->getAttributes();

    if (isset($attributes)) {
      if (!empty($attributes[$attribute][0])) { <-- Evaluates to FALSE
        return $attributes[$attribute][0];
      }
    }

    throw new SimplesamlphpAttributeException(sprintf('Error in simplesamlphp_auth.module: no valid "%s" attribute set.', $attribute));
}

Proposed resolution

Allow other modules to handle the sync. I use an event subscriber to listen to the ExternalAuthEvents::REGISTER event, at which point I can make an external API call to retrieve the user's email (and name, age, etc.). A simple checkbox on the configuration page would allow an admin to override the default behavior.