Add composite fields to search API (#2070)

Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>

Author: MichaelOpitz2

output/openapi/elasticsearch-openapi.json

@@ -24,6 +24,8 @@ import { Script } from '@_types/Scripting'
 export type RuntimeFields = Dictionary<Field, RuntimeField>
 
 export class RuntimeField {
+  /** For type `composite` */
+  fields?: Dictionary<string, CompositeSubField>
   /** For type `lookup` */
   fetch_fields?: RuntimeFieldFetchFields[]
   /**
@@ -47,6 +49,10 @@ export class RuntimeField {
   type: RuntimeFieldType
 }
 
+class CompositeSubField {
+  type: RuntimeFieldType
+}
+
 /** @shortcut_property field */
 export class RuntimeFieldFetchFields {
   field: Field