Summary

Adds a new `NagiosProvider` that integrates Keep with Nagios monitoring systems.

• Supports Nagios XI (commercial) via REST API (`/nagiosxi/api/v1/objects/hoststatus` and `servicestatus`) using API key authentication
• Supports Nagios Core (open source) via CGI JSON API (`statusjson.cgi`) using HTTP basic auth
• Supports webhook push notifications via custom Nagios notification commands (full setup guide in `webhook_markdown`)
• Maps Nagios host states (UP/DOWN/UNREACHABLE) and service states (OK/WARNING/CRITICAL/UNKNOWN) to Keep `AlertStatus` and `AlertSeverity`
• Both `_get_alerts()` (pull) and `_format_alert()` (push) are implemented
• Includes `README.md` with setup instructions for both Nagios XI and Core
• Includes `alerts_mock.py` with sample webhook payload
• Includes `tests/providers/nagios_provider/` with 20 unit tests covering all state mappings and edge cases

Files Changed

• `keep/providers/nagios_provider/init.py` (empty, required)
• `keep/providers/nagios_provider/nagios_provider.py` (provider implementation)
• `keep/providers/nagios_provider/README.md` (setup guide)
• `keep/providers/nagios_provider/alerts_mock.py` (sample webhook payload)
• `tests/providers/nagios_provider/test_nagios_provider.py` (20 unit tests)

Test plan

• `_format_alert()` correctly handles HOST + SERVICE webhook payloads
• All Nagios states (DOWN/UNREACHABLE/UP for hosts; CRITICAL/WARNING/UNKNOWN/OK for services) map correctly to Keep `AlertStatus` and `AlertSeverity`
• Alert IDs generated deterministically from hostname/service name
• Missing optional fields do not raise exceptions
• Class name `NagiosProvider` matches Keep’s provider factory convention
• Both Nagios XI (API key) and Nagios Core (basic auth) code paths implemented
• README documents webhook setup for both Nagios XI and Nagios Core

Closes #3960

/claim #3960

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