Closes #3810

📑 Description

Integrated Icinga2 Provider for Keep. Will enable users to receive push-based monitoring alerts directly into Keep from Icinga2.

Note: PR improves on closed [#4604]

• Handling incoming push alerts via webhooks from Icinga2’s event command system
• Payload parsing supporting both host & service alerts
• Mapping between Icinga2 states & Keep alert severities/statuses
• Webhook-first design
• Follows the BaseProvider API consistent with all other Keep’s providers

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

• Relevant Icinga2 Metadata preservation in Keep alert format
• Webhook endpoint for Icinga2 event commands; enables push-based alert delivery (no polling)
• Icinga’s Icon Added

/claim #3810