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

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

* Fix review comments.

app/data_retention.go

@@ -0,0 +1,18 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func (a *App) GetDataRetentionPolicy() (*model.DataRetentionPolicy, *model.AppError) {
+	if a.DataRetention == nil {
+		return nil, model.NewAppError("App.GetDataRetentionPolicy", "ent.data_retention.generic.license.error", nil, "", http.StatusNotImplemented)
+	}
+
+	return a.DataRetention.GetPolicy()
+}