Add db_iam_username variable for PostgreSQL IAM authentication

This variable allows specifying a separate database username for IAM
authentication when postgres_enable_iam_auth=true. If null, it uses
the db_username (master user).

Required for passwordless PostgreSQL authentication tests.

Author: raviharshicorp

variables.tf

@@ -222,6 +222,12 @@ variable "db_username" {
   description = "PostgreSQL instance username. No special characters."
 }
 
+variable "db_iam_username" {
+  default     = null
+  type        = string
+  description = "PostgreSQL IAM username for TFE connection when IAM auth is enabled. If null, uses db_username. No special characters."
+}
+
 variable "db_backup_retention" {
   type        = number
   description = "The days to retain backups for. Must be between 0 and 35"