Revisit Resource Functions Returning Service Objects Functionality

[ThisaruGuruge]

Currently, the Ballerina GraphQL module supports resource functions that return dynamic service objects. But as per the discussion with James, we should not support this. Instead, we should support typed services only.

For example, the following should not be allowed:

import ballerina/graphql;

service graphql:Service on new graphql:Listener(9090) {
    resource function get profile() returns service object {} {
        return new service object {
            resource function get name() returns string {
                return "Sherlock";
            }
        }
    }
}

Instead, when a user needs to return service from a resource function, they should define a service type.