[Drupal 8] Override id map with custom plugin for migrations

id_map_custom.php

<?php

/**
 * Implements hook_migration_plugins_alter().
 */
function my_module_migration_plugins_alter(array &$migrations) {
  foreach ($migrations as $id => $configuration) {
    if (!empty($migrations[$id]['idMap'])) {
      // Do not override existing values.
      continue;
    }
    // Override id map for each migration.
    $migrations[$id]['idMap'] = [
      'plugin' => 'sql_large_key',
    ];
  }
}