Add PNA support to Service Directory module

Endpoints in Service Directory can be *associated* with a
VPC. In this case, they can be used by supported Google
Cloud products to send requests directly to resources inside
a VPC. This feature is called Private Network Access.

The `google_service_directory_endpoint` resource supports
this configuration with a new argument `network`.
Unfortunately, this argument has an unusual format: it
is similar to a standard VPC ID, but instead of the project ID,
it expects the project number.

modules/service-directory/main.tf

@@ -75,4 +75,5 @@ resource "google_service_directory_endpoint" "default" {
   metadata    = try(var.endpoint_config[each.key].metadata, null)
   address     = try(var.endpoint_config[each.key].address, null)
   port        = try(var.endpoint_config[each.key].port, null)
+  network     = try(var.endpoint_config[each.key].network, null)
 }