add listBucket internal to validator lambda

Author: alexnuttall

infrastructure/terraform/modules/backend-api/module_lambda_validate_letter_template_files.tf

@@ -61,6 +61,18 @@ data "aws_iam_policy_document" "validate_letter_template_files" {
     resources = ["${module.s3bucket_internal.arn}/*"]
   }
 
+  statement {
+    sid    = "AllowS3InternalList"
+    effect = "Allow"
+
+    actions = [
+      "s3:ListBucket",
+      "s3:ListBucketVersions",
+    ]
+
+    resources = [module.s3bucket_internal.arn]
+  }
+
   statement {
     sid    = "AllowSQSEventSource"
     effect = "Allow"