make sure domain_name is exported from lambda_with_api_gateway for use by fast-api

infrastructure/terraform/modules/lambda_with_api_gateway/outputs.tf

@@ -4,4 +4,8 @@ output "role_name" {
 
 output "api_endpoint" {
   value = aws_apigatewayv2_stage.this.invoke_url
+}
+
+output "domain_name" {
+  value = var.domain_name
 }