This PR implements a new Nagios Provider for Keep that allows ingesting alerts from Nagios monitoring system.

Features

• NRDP API Support: Full support for the Nagios Remote Data Processor (NRDP) REST API
• CGI API Support: Legacy support for Nagios Core CGI API with username/password authentication
• Host Alerts: Ingest host status alerts (DOWN, UNREACHABLE)
• Service Alerts: Ingest service status alerts (WARNING, CRITICAL, UNKNOWN)
• Severity Mapping: Maps Nagios states to Keep severity levels
• Acknowledgment Tracking: Tracks whether alerts have been acknowledged in Nagios
• Web Interface Links: Provides direct links to Nagios web interface for each alert

Configuration

NRDP API (Recommended)

authentication:
  host_url: "https://nagios.example.com"
  nrdp_token: "your-nrdp-token"
  api_type: "nrdp"

CGI API (Legacy)

authentication:
  host_url: "https://nagios.example.com"
  username: "admin"
  password: "secret"
  api_type: "cgi"

Testing

Includes comprehensive unit tests for:

• Configuration validation
• Scope validation
• Alert retrieval and parsing
• Error handling

Fixes #3960

/claim #3960