Postgres - Support GIN and GIST

Closes #3397622

core/lib/Drupal/Core/Database/Schema/Index.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Core\Database\Schema;
+
+/**
+ * Class for Schema API index specifications containing db-specific config.
+ */
+final class Index extends \ArrayObject {
+
+  /**
+   * Constructor.
+   *
+   * @param array $fields
+   *   Array of field specifications for the index.
+   * @param array $config
+   *   Array of db-specific config, keyed by database type. Acceptable
+   *   keys/values for the configuration are determined by the database driver.
+   *
+   * @see \Drupal\Core\Database\Connection::databaseType()
+   */
+  public function __construct(array $fields, protected array $config) {
+    parent::__construct($fields);
+  }
+
+  /**
+   * Getter for db-specific configuration.
+   *
+   * @param string $database_type
+   *   Database type.
+   *
+   * @return array
+   *   Configuration; if not set, will return an empty array.
+   */
+  public function getDatabaseConfig(string $database_type): array {
+    return $this->config[$database_type] ?? [];
+  }
+
+}