Closes #3960

📑 Description

Introduces Nagios as a new supported provider. Enables receptions of alerts from Nagios within Keep. The integration is designed to be push-based and relies on Nagios’s native Event Handler system.

• new provider class NagiosProvider to handle incoming webhook requests from Nagios.
• functionality to parse JSON payloads sent by Nagios event handler and map it to Keep’s standard AlertDto format.
• detailed documentation for Nagios-side config for the provider
• Nagios Logo added in iconography.

✅ 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

/claim #3960