allow for configurable SKUs on Azure Firewall

[glennmusa]

Description

Currently we hardcode Azure Firewall to Premium when it is a subscription-scope enabled private preview feature

missionlz/src/modules/firewall/main.tf

Lines 27 to 32 in 08eae8e

	resource "azurerm_firewall" "firewall" {
	name = var.saca_fwname
	location = data.azurerm_resource_group.hub.location
	resource_group_name = data.azurerm_resource_group.hub.name
	sku_tier = "Premium"
	tags = var.tags

Steps to Reproduce
Steps to reproduce the behavior:

1. Apply a firewall module in a subscription without Azure Firewall Premium

Expected behavior
I should be able to deploy the saca-hub configuration in any subscription regardless of preview features

Actual behavior
I can't deploy saca-hub in a subscription that isn't enrolled in the Azure Firewall Premium preview