Add support for cross project NEGs (#3215)

* Added cross project NEG support
* Made neg_configs variable same as in net-lb-app-int
* Added support for cross-project negs in net-lb-app-ext-regional module


---------

Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>
Co-authored-by: Ludovico Magnocavallo <ludo@qix.it>
Co-authored-by: Julio Castillo <juliocc@gmail.com>
Co-authored-by: Wiktor Niesiobędzki <wiktorn@google.com>

modules/net-lb-app-ext-regional/negs.tf

@@ -151,7 +151,11 @@ resource "google_compute_region_network_endpoint_group" "psc" {
 
 resource "google_compute_region_network_endpoint_group" "serverless" {
   for_each = local.neg_regional_serverless
-  project  = var.project_id
+  project = (
+    each.value.project_id == null
+    ? var.project_id
+    : each.value.project_id
+  )
   region = try(
     each.value.cloudrun.region, each.value.cloudfunction.region, null
   )

modules/net-lb-app-ext-regional/variables.tf

@@ -66,6 +66,7 @@ variable "name" {
 variable "neg_configs" {
   description = "Optional network endpoint groups to create. Can be referenced in backends via key or outputs."
   type = map(object({
+    project_id  = optional(string)
     description = optional(string)
     cloudfunction = optional(object({
       region          = string