adds specifications for project_routing in the body of open pit request (#5754)

Author: piergm

output/schema/schema.json

@@ -140,5 +140,17 @@ export interface Request extends RequestBase {
      * Filter indices if the provided query rewrites to `match_none` on every shard.
      */
     index_filter?: QueryContainer
+    /**
+     * Specifies a subset of projects to target for the PIT request using project
+     * metadata tags in a subset of Lucene query syntax.
+     * Allowed Lucene queries: the _alias tag and a single value (possibly wildcarded).
+     * Examples:
+     *  _alias:my-project
+     *  _alias:_origin
+     *  _alias:*pr*
+     * Supported in serverless only.
+     * @availability serverless stability=stable visibility=feature_flag feature_flag=serverless.cross_project.enabled
+     */
+    project_routing?: ProjectRouting
   }
 }