Implements Nagios provider support for Keep via webhook ingestion.

/claim #3960 Closes #3960

What’s included

• Added new provider: keep/providers/nagios_provider/nagios_provider.py
• Added webhook-oriented parsing for common Nagios payload variants (Core/XI macro styles)
• Added host/service state mappings to AlertSeverity and AlertStatus
• Added notification type handling (PROBLEM, RECOVERY, ACKNOWLEDGEMENT, downtime/flapping related notifications)
• Added provider docs: keep/providers/nagios_provider/README.md
• Added mock payload: keep/providers/nagios_provider/alerts_mock.py
• Added formatter tests: tests/test_nagios_provider.py

Notes

• This implementation focuses on webhook-based ingestion and normalization for Nagios notifications.
• It keeps the patch scoped and compatible with Keep’s dynamic provider loading pattern.