fix(url-loader): graphql v15 support

Author: ardatan

.changeset/huge-boxes-say.md

@@ -0,0 +1,5 @@
+---
+'@graphql-tools/url-loader': patch
+---
+
+Fix GraphQL v15 support

packages/loaders/url/src/index.ts

@@ -1,4 +1,4 @@
-import { buildASTSchema, buildSchema, IntrospectionOptions } from 'graphql';
+import { buildASTSchema, buildSchema, GraphQLSchema, IntrospectionOptions } from 'graphql';
 import WebSocket from 'isomorphic-ws';
 import { buildGraphQLWSExecutor } from '@graphql-tools/executor-graphql-ws';
 import {
@@ -15,6 +15,7 @@ import {
   BaseLoaderOptions,
   ExecutionRequest,
   Executor,
+  getDefinedRootType,
   getOperationASTFromRequest,
   isUrl,
   Loader,
@@ -431,6 +432,13 @@ export class UrlLoader implements Loader<LoadFromUrlOptions> {
         executor,
         batch: options?.batch,
       });
+      if (!('getRootType' in source.schema)) {
+        Object.defineProperty(source.schema, 'getRootType', {
+          value(this: GraphQLSchema, operation: any) {
+            return getDefinedRootType(this, operation);
+          },
+        });
+      }
     }
 
     return [source];