chore(ec2): add AppSync endpoint constant (#25694)

On May 3rd AppSync announced Private API support: https://aws.amazon.com/about-aws/whats-new/2023/05/aws-appsync-graphql-apis-private-api-support/

This PR adds the interface endpoint constant for AppSync.

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

packages/aws-cdk-lib/aws-ec2/lib/vpc-endpoint.ts

@@ -266,6 +266,7 @@ export class InterfaceVpcEndpointAwsService implements IInterfaceVpcEndpointServ
   public static readonly APP_MESH = new InterfaceVpcEndpointAwsService('appmesh-envoy-management');
   public static readonly APP_RUNNER = new InterfaceVpcEndpointAwsService('apprunner');
   public static readonly APP_RUNNER_REQUESTS = new InterfaceVpcEndpointAwsService('apprunner.requests');
+  public static readonly APP_SYNC = new InterfaceVpcEndpointAwsService('appsync-api');
   public static readonly APPLICATION_MIGRATION_SERVICE = new InterfaceVpcEndpointAwsService('mgn');
   public static readonly APPSTREAM_API = new InterfaceVpcEndpointAwsService('appstream.api');
   public static readonly APPSTREAM_STREAMING = new InterfaceVpcEndpointAwsService('appstream.streaming');