Fix backward compatibility for NewNetworkServiceEndpointRegistryClient and NewNetworkServiceRegistryClient

[edwarnicke]

NewNetworkServiceEndpointRegistryClient and NewNetworkServiceRegistryClient where recently enhanced. In that process, the leading "connectTo *url.URL" parameter was removed. The functionality continues to be supported via the WithClientURL option.

In order to avoid breaking current downstream users, the leading "connectTo *url.URL" has been restored in such a way that it has no effect if it is nil and otherwise utilize the WithClient option (to insure the code path is exercised).

[edwarnicke]

@denis-tingaikin Totally fine to discuss on the community call :) My concern is wrt folks who have their own NSEs and NSCs and the change this would mean for them.