PLT-7705: API to get data retention policy. (#7539)

* PLT-7705: API to get data retention policy.

* Fix review comments.

api4/data_retention_test.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api4
+
+import (
+	"testing"
+)
+
+func TestDataRetentionGetPolicy(t *testing.T) {
+	th := Setup().InitBasic()
+	defer TearDown()
+
+	_, resp := th.Client.GetDataRetentionPolicy()
+	CheckNotImplementedStatus(t, resp)
+}