Add Same as translation to the Xliff Target content options

Here is a first approach for this, without tests. Presumes that Same as source is the fallback if no translation is available yet.

Patch above requires at least to review SourcePluginInterface::getData or perhaps introduce getTranslationData will continue on this a bit later, currently reviewing how to integrate with #2978341: Support pending revisions and accepting translation as a specific moderation state

About the content moderation integration, if we continue with the initial approach, we could change the ContentEntitySource::loadMultiple() from https://www.drupal.org/project/tmgmt/issues/2978341#comment-13209387
like this to get the latest translated revision, using EntityRepository::getActiveMultiple() with a language context.

public static function loadMultiple($entity_type_id, array $entity_ids, $langcode = NULL) {
    /** @var \Drupal\Core\Entity\EntityStorageInterface $storage */
    $storage = \Drupal::entityTypeManager()->getStorage($entity_type_id);
    // Load the latest revision if the entity type is revisionable.
    if ($storage->getEntityType()->isRevisionable() && $storage instanceof TranslatableRevisionableStorageInterface) {
      /** @var \Drupal\Core\Entity\EntityRepositoryInterface $entityRepository */
      $entityRepository = \Drupal::service('entity.repository');
      if (!is_null($langcode)) {
        $dataType = 'language';
        $contextIdPrefix = '@language.current_language_context:';
        $contexts = [
          $contextIdPrefix . LanguageInterface::TYPE_CONTENT => new Context(new ContextDefinition($dataType), $langcode),
          $contextIdPrefix . LanguageInterface::TYPE_INTERFACE => new Context(new ContextDefinition($dataType), $langcode),
        ];
        $activeVariants = $entityRepository->getActiveMultiple($entity_type_id, $entity_ids, $contexts);
      }
      else {
        $activeVariants = $entityRepository->getActiveMultiple($entity_type_id, $entity_ids);
      }
      foreach ($activeVariants as $variant) {
        $entities[$variant->id()] = $variant;
      }
    }
    else {
      $entities = $storage->loadMultiple($entity_ids);
    }
    return $entities;
}

I will also continue to look a the proposed implementation with checkboxes so the site builder can choose to include the source or not in the initial translation.

Implementation with checkboxes: allows
- empty
- source
- translation
- translation with fallback to source

Also fixes the target configuration that was not displayed on the form after save.

Still requires
- configuration update from single valued to multi valued configuration for 'target' (copy old configuration and initialise new - update or post update hook of the tmgmt_file module)
- tests