Add support for serverless NEGs to ILB L7 module

[ludoo]

ILB L7 only supports Cloud Run at the moment, but it will be fairly easy to add support for other serverless NEG types in the future.

PSC NEGs support is also made simple by this change, it will be added in a future PR.

This change also re-enables the forwarding rule output, which was mistakenly commented out in the refactor (thanks Daniel for pointing this out).

Serverless NEG example from the README:

module "ilb-l7" {
  source     = "./fabric/modules/net-ilb-l7"
  name       = "ilb-test"
  project_id = var.project_id
  region     = "europe-west1"
  backend_service_configs = {
    default = {
      backends = [{
        balancing_mode = "RATE"
        group = "my-neg"
        max_rate       = { per_endpoint = 1 }
      }]
    }
  }
  neg_configs = {
    my-neg = {
      cloudrun = {
        region = "europe-west1"
        target_service = {
          name = "my-run-service"
        }
      }
    }
  }
  vpc_config = {
    network    = var.vpc.self_link
    subnetwork = var.subnet.self_link
  }
}