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


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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/variables.tf

@@ -115,6 +115,7 @@ 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