d6_cck_file process plugin now takes optional 'migration' option

The d6_cck_file process plugin processes Drupal 6 file fields into Drupal 8 file entity reference fields, in part by translating the Drupal 6 file ID (fid) into the equivalent Drupal 8 fid. Through Drupal 8.2.1, the ID of the file migration used to translate the fid, 'd6_file', was hardcoded in the process plugin - if implementing a custom Drupal 6 migration process with a different ID for the file migration, you would need to write a custom process plugin extending d6_cck_file to properly map the IDs. With Drupal 8.2.2, you can simply provide a 'migration' option pointing to your custom file migration.

Previously:
my_node_migration.yml:

...
  process:
...
    field_image:
      plugin: my_cck_file
      source: field_image
...

MyCckFile.php:

namespace Drupal\my_migrate_module\Plugin\migrate\process;

use Drupal\file\Plugin\migrate\process\d6\CckFile;
use Drupal\migrate\Plugin\MigrationInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * @MigrateProcessPlugin(
 *   id = "my_cck_file"
 * )
 */
class MyCckFile extends CckFile {

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
    $migration_plugin_configuration = [
      'source' => ['fid'],
      'migration' => 'my_file_migration',
    ];

    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $migration,
      $container->get('plugin.manager.migrate.process')->createInstance('migration', $migration_plugin_configuration, $migration)
    );
  }

}

Now:

...
  process:
...
    field_image:
      plugin: d6_cck_file
      source: field_image
      migration: my_file_migration
...