[8.15] Use an enum for ES|QL format parameter (#2873) (#2884)

Author: pquentin

output/openapi/elasticsearch-openapi.json

@@ -33,7 +33,7 @@ export interface Request extends RequestBase {
     /**
      * A short version of the Accept header, e.g. json, yaml.
      */
-    format?: string
+    format?: EsqlFormat
     /**
      * The character to use between values within a CSV row. Only valid for the CSV format.
      */
@@ -63,3 +63,14 @@ export interface Request extends RequestBase {
     query: string
   }
 }
+
+export enum EsqlFormat {
+  csv,
+  json,
+  tsv,
+  txt,
+  yaml,
+  cbor,
+  smile,
+  arrow
+}