Add an index.
$table: The table to be altered.
$name: The name of the index.
$fields: An array of field names.
Drupal\Core\Database\SchemaObjectDoesNotExistException If the specified table doesn't exist.
Drupal\Core\Database\SchemaObjectExistsException If the specified table already has an index by that name.
public abstract function addIndex($table, $name, $fields);