add HTTP support for sia agents for health checks

[havetisyan]

Description

addresses issue #3136

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

Hello @havetisyan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Athenz SIA agent by adding HTTP-based health check capabilities. It introduces a mechanism to track the refresh status and validity of all managed certificates, exposing this information through dedicated HTTP endpoints. This change aims to provide better operational visibility into the agent's health and the state of its certificates, addressing issue #3136.

Highlights

• HTTP Health Check Server: Introduced a new HTTP server within the SIA agent to provide health check endpoints, allowing external systems to monitor the agent's status.
• Certificate Status Tracking: Implemented a StatusTracker to monitor the refresh status of both service and role certificates, recording success, failure, and expiry times.
• Health Endpoints: Added two HTTP endpoints: /status which returns 200 OK if all certificates are valid (not expired) and 400 Bad Request otherwise, and /certs which provides detailed JSON output of all tracked certificate statuses.
• Configurable HTTP Port: The HTTP server port is now configurable via the ATHENZ_SIA_HTTP_PORT environment variable or the HttpPort field in the agent's configuration.
• Enhanced Observability: Integrated the StatusTracker into the certificate refresh logic, ensuring that every certificate refresh attempt (success or failure) is recorded, improving the agent's observability.

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Code Review

This pull request introduces HTTP health check support for SIA agents by adding a new http package with a StatusTracker to monitor certificate refresh status. The agent is updated to utilize this tracker and launch an HTTP server. The implementation is solid and includes thorough testing. My review includes suggestions to improve efficiency by avoiding redundant function calls within loops, enhance error handling by logging a previously ignored error, use a more semantically correct HTTP status code for health check failures, and improve thread safety in the GetCerts function by performing a deep copy to prevent potential race conditions.