Allow SSO authentication while maintenance mode is enabled

The feature request is quite simple: I would like to be able to login using SSO while maintenance mode is enabled.

I am not really sure of the implementation direction:

Option 1: Default Drupal core login method is always available while maintenance mode is enabled so it looks decent to assume the same for SSO login method. If that is the direction, it is simply about adding the _maintenance_access option to TRUE for samlauth.saml_controller_login, samlauth.saml_controller_logout and samlauth.saml_controller_acs in samlauth.routing.yml. (I added logout to the route list to be consistent with user.routing.yml). I will provide a patch for this option.

Option 2: If we say that it should be an option but not the default behavior, we then need to alter routes if the option is set. I have not worked on making it configurable but the route subscriber would look like:

namespace Drupal\samlauth\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Listens to the dynamic route events.
 */
class RouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection) {
    $route_names = [
      'samlauth.saml_controller_login',
      'samlauth.saml_controller_acs',
      'samlauth.saml_controller_logout',
    ];

    foreach ($route_names as $route_name) {
      if ($route = $collection->get($route_name)) {
        $route->setOption('_maintenance_access', TRUE);
      }
    }
  }

}