External uses should be checked only against 'cas' provider

[claudiu-cristea]

In \Drupal\oe_authentication\Access\ExternalUserAccessCheck::access() there is this piece of code:

if ($this->authMap->getAll($account->id()) === []) {
  return AccessResult::allowed();
}

Using ::getAll() here is wrong because on a system might offer support for multiple external authentication authorities (social media, Github, OpenID, etc.)

Even if limiting the access to those routes could be useful when using other provider/authority, this module should not make assumptions on behalf them.