Add private-traffic-only option to GCP Private Service Connect enabled deployments

[abhinavg6]

We would like to add an option to enable private only traffic to a deployment i.e. the traffic would be allowed only via GCP private endpoints and not over the public network. If the option is not enabled i.e. in the default mode, both private and public traffic should be allowed, where the public traffic could be restricted to specific sources using IP allowlisting.

If the investigation indicates that this is not possible just via Terraform, then it would be good to produce suggestions for how this could be implemented across Terraform and the core application.