Implement catalog zones options new syntax based on custom properties

According to DNS catalog zones draft version 5 document, catalog
zone custom properties must be placed under the "ext" label.

Make necessary changes to support the new custom properties syntax in
catalog zones with version "2" of the schema.

Change the default catalog zones schema version from "1" to "2" in
ARM to prepare for the new features and changes which come starting
from this commit in order to support the latest DNS catalog zones draft
document.

Make some restructuring in ARM and rename the term catalog zone "option"
to "custom property" to better reflect the terms used in the draft.

Change the version of 'catalog1.zone.' catalog zone in the "catz" system
test to "2", and leave the version of 'catalog2.zone.' catalog zone at
version "1" to test both versions.

Add tests to check that the new syntax works only with the new schema
version, and that the old syntax works only with the legacy schema
version catalog zones.

bin/tests/system/catz/ns3/catalog.example.db.in

@@ -0,0 +1,14 @@
+; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+;
+; SPDX-License-Identifier: MPL-2.0
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0.  If a copy of the MPL was not distributed with this
+; file, you can obtain one at https://mozilla.org/MPL/2.0/.
+;
+; See the COPYRIGHT file distributed with this work for additional
+; information regarding copyright ownership.
+
+@ 3600 SOA . . 1 86400 3600 86400 3600
+@ 3600 IN NS invalid.
+version IN TXT "1"