MM-21898 - Part 1: Generate and use an interface instead of *A… (#13840)

* Generate and use an interface instead of *App

app/layer_generators/app_iface.go.tmpl

@@ -0,0 +1,12 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+// Code generated by "make store-layers"
+// DO NOT EDIT
+
+package app
+
+// AppIface is extracted from App struct and contains all it's exported methods. It's provided to allow partial interface passing and app layers creation.
+type AppIface interface {
+	{{.Content}}
+}