When I run this URL in browser, it prompts the IS sign-in popup windows if I am not logging in the IS yet. Once I sign in, I am able to get the response.
So how can I have the restful web services to be consumed by the client without being needed to sign in the IS?
It is not recommended and not a best practice to expose the service to the client/consumer without login access (anonymous) if you want to restrict the IS access and only allow access to the rest resource, create user, group and acl. Set this acl to the service (refer IS administration guide for more details)