Added conditional project designation feature for relevant modules.

modules/dns-managed-zone/variables.tf

@@ -1,6 +1,8 @@
 variable "project_id" {
-  description = "The project ID where the managed zone will be created."
+  description = "The GCP project ID. If not provided, the default project will be used."
   type        = string
+  nullable    = true
+  default     = null
 }
 
 variable "dns_name" {