Add ES|QL endpoint for Serverless (#2560)

Author: luigidellaquila

specification/esql/query/QueryRequest.ts

@@ -26,6 +26,7 @@ import { EsqlVersion } from '@esql/_types/EsqlVersion'
  * Executes an ES|QL request
  * @rest_spec_name esql.query
  * @availability stack since=8.11.0 stability=experimental
+ * @availability serverless stability=experimental visibility=public
  * @doc_id esql-query
  */
 export interface Request extends RequestBase {