Generating sitemap variants in 3.x does only generate sitemap types

Added additional logic so new sitemap variants are created next to the sitemap types.
The sitemap variant automatically uses the newly created sitemap type.

Also fixed issue that \Drupal::entityTypeManager() was being used instead of the injected service.

This also fixes the issue described here: https://www.drupal.org/project/domain_simple_sitemap/issues/3417078 🐛 Multi-Site using Domain Access: Links added from other domains RTBC
The entity_url generator was added by default to all sitemap types being generated causing any url being added to the sitemaps. It should suffice to only use domain_entity_url and if for a specific case you really want a sitemap with all entities added to the sitemap you could add it.
I'm not really seeing the benefit of doing that as the domain name in which it is added would often be wrong.

Committed. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.