Add a way to configure allowed entity types for workspace-specific config

There is still a problem with the linkcheckerlink config entity type which is used by the Link Checker module. When a workspace is active, and a node with a link in an HTML text field is being saved, the following error occurs:

Drupal\Core\Entity\EntityStorageException: The "linkcheckerlink" entity type can only be saved in the default workspace. in Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 817 of core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).

The linkcheckerlink config entity type needs to be added to the list of option is the Enabled config entity types multi-select field in the Workspace Settings page. I was able to get around this error by marking linkcheckerlink config entity type as "internal" inside hook_entity_type_build():

function my_module_entity_type_build(array &$entity_types) {
  $entity_type_id = 'linkcheckerlink';
 
  if (isset($entity_types[$entity_type_id]) && $entity_types[$entity_type_id] instanceof EntityTypeInterface) {
    /** @var \Drupal\Core\Entity\EntityTypeInterface $entity_type */
    $entity_type = $entity_types[$entity_type_id];
 
    // Mark the entity type as internal.
    $entity_type->set('internal', TRUE);
  }
}