[Backport 8.13] Define Pipeline._meta as optional (#2438)

* Define `Pipeline._meta` as optional

* Run make contrib

---------

Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>
(cherry picked from commit 0672f8f)

Co-authored-by: Pierre Gayvallet <pierre.gayvallet@gmail.com>

Author: github-actions[bot]

output/openapi/elasticsearch-serverless-openapi.json

@@ -41,7 +41,7 @@ export class Pipeline {
   /**
    * Arbitrary metadata about the ingest pipeline. This map is not automatically generated by Elasticsearch.
    */
-  _meta: Metadata
+  _meta?: Metadata
 }
 
 // Unused .. but let's keep it for now