Add forced NOTIFY feature for DNS zones

- Introduced `custom_field_force_notify` setting to trigger immediate NOTIFY to a zone's nameservers when its SOA serial number changes and the custom field is set to True.
- Updated README to document the new feature and its configuration.
- Refactored notify handler to include a new job for sending forced NOTIFY messages directly to zone nameservers.
- Enhanced zone signal handling to check for SOA serial changes and trigger forced NOTIFY accordingly.

CHANGELOG.md

@@ -1,3 +1,11 @@
+## Unreleased
+- Add `custom_field_force_notify` setting. When set to the name of a boolean custom field on the
+  NetBox DNS Zone object, the plugin sends a NOTIFY directly to a zone's own nameservers whenever
+  the zone has that custom field set to `True` and its SOA serial number changes. Nameserver
+  addresses are resolved from NetBox DNS A/AAAA records within the zone's view and the NOTIFY is
+  signed with the per-view TSIG key. A companion `force_notify_port` setting (default 53) controls
+  the destination port.
+
 ## 1.0.7 - 2026-03-02
 README Change - Moving private keys to global scope since Bind 9.20 view scoped keys have become unreliable and sometimes wouldnt match.