add Backup for GKE support

tests/modules/gke_cluster/examples/backup.yaml

@@ -0,0 +1,28 @@
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+values:
+  module.cluster-1.google_container_cluster.cluster:
+    backup_configs:
+      enable_backup_agent: true
+      backup_plans:
+        backup-1:
+          region: europe-west2
+          schedule: 0 9 * * 1
+          retention_policy_days: 30
+          retention_policy_lock: false
+          retention_policy_delete_lock_days: 15
+
+counts:
+  google_container_cluster: 1