Improving usability of module

@aleevas Let's run this part in batch (separated batch operation for each active domain):

public function generateDomainSitemapVariants(array &$form, FormStateInterface $form_state) {
  $active_domains = $this->domainSitemapManager->setSitemapVariantsForEnabledDomain();
  if (!empty($active_domains)) {
    foreach ($active_domains as $active_domain) {
      $message = $this->t('Sitemap variant was created for domain @domain', [
          '@domain' => $active_domain->label()
      ]);
      \Drupal::messenger()->addMessage($message);
    }
  }
}

+++ b/domain_simple_sitemap.services.yml
@@ -2,4 +2,7 @@ services:
+domain_simple_sitemap.manager:

This is incorrect spacing - this line should be indented.

Here is a fixed patch, I hope.

Looks ok, tested