fix(appsync): multiple interface types generates invalid schema (#10481)

Make the GraphQL Schema construct multiple interface implementation in the following manner:

```gql
type Object implements Interface1 & interface2 {
  ...
}
```

Fixes #10479

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-appsync/lib/private.ts

@@ -202,7 +202,7 @@ function concatAndDedup<T>(left: T[], right: T[]): T[] {
 function generateInterfaces(interfaceTypes?: InterfaceType[]): string {
   if (!interfaceTypes || interfaceTypes.length === 0) return '';
   return interfaceTypes.reduce((acc, interfaceType) =>
-    `${acc} ${interfaceType.name},`, ' implements').slice(0, -1);
+    `${acc} ${interfaceType.name} &`, ' implements').slice(0, -2);
 }
 
 /**

packages/@aws-cdk/aws-appsync/test/appsync-object-type.test.ts

@@ -61,7 +61,7 @@ describe('testing Object Type properties', () => {
     api.addType(objectTest);
 
     const gql_interface = 'interface baseTest {\n  id: ID\n}\ninterface anotherTest {\n  id2: ID\n}\n';
-    const gql_object = 'type objectTest implements anotherTest, baseTest {\n  id3: ID\n  id2: ID\n  id: ID\n}\n';
+    const gql_object = 'type objectTest implements anotherTest & baseTest {\n  id3: ID\n  id2: ID\n  id: ID\n}\n';
     const out = `${gql_interface}${gql_object}`;
 
     // THEN