Per PR review feedback:
- Rename settings: custom_field_force_notify -> notify_ns_custom_field_name,
force_notify_port -> notify_ns_port.
- Add notify_ns_all_zones (bool, default False) to enable the feature globally
for every zone; it coexists with the per-zone custom field — a NOTIFY is sent
if the global switch is on OR the zone's custom field is set.
- Rename the internal job/scheduler/log strings to the notify_ns naming for
consistency (SendZoneNSNotify, schedule_notify_ns).
- Update README and CHANGELOG.
- 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.
d_ev