lb_external module should support network_tier

[jochristian]

Is your feature request related to a problem?

Seems like the lb_external module is "half-finished" on network_tier.
The variable exists but it's not added to main.tf

variable "network_tier" {
description = "The networking tier used for configuring this address. If this field is not specified, it is assumed to be PREMIUM. Possible values are PREMIUM and STANDARD."

Describe the solution you'd like

Add this to main.tf. Under google_compute_forwarding_rule

Create forwarding rule for each specified rule

resource "google_compute_forwarding_rule" "rule" {
for_each = var.rules

name = each.key
project = var.project
region = local.region
network_tier = var.network_tier <<<-- This should be added.

Describe alternatives you've considered.

No response

Additional context

No response