Add cross project support for backend bucket (#3273)

* Added cross project support for backend bucket.

* Ran terraform fmt.

* Ran tfdoc.

---------

Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>

modules/net-lb-app-ext/backends.tf

@@ -17,8 +17,12 @@
 # tfdoc:file:description Backend groups and backend buckets resources.
 
 resource "google_compute_backend_bucket" "default" {
-  for_each                = var.backend_buckets_config
-  project                 = var.project_id
+  for_each = var.backend_buckets_config
+  project = (
+    each.value.project_id == null
+    ? var.project_id
+    : each.value.project_id
+  )
   name                    = "${var.name}-${each.key}"
   bucket_name             = each.value.bucket_name
   compression_mode        = each.value.compression_mode

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

@@ -23,6 +23,7 @@ variable "backend_buckets_config" {
     description             = optional(string)
     edge_security_policy    = optional(string)
     enable_cdn              = optional(bool)
+    project_id              = optional(string)
     cdn_policy = optional(object({
       bypass_cache_on_request_headers = optional(list(string))
       cache_mode                      = optional(string)