feat: implemented alerting for expiring certificates for application gateway

[nc-shahidazim]

Description

This update introduces alerting for expiring SSL certificates on Application Gateway by leveraging the existing Application Insights availability test framework. The enhancement extends the current test capability to validate certificate expiry and provides flexible options for custom headers and request body.

Context

Application Gateway does not provide native certificate‑expiry alerting through standard monitoring signals. To address this limitation, the existing availability test mechanism is extended to validate SSL certificate expiry on each probe execution. This ensures consistent monitoring without introducing new resources or pipelines, while improving detection of upcoming certificate failures.

What’s included

• Implemented SSL certificate expiry checks using Application Insights availability tests.
• Added support for custom headers, enabling scenarios where routing, authentication tokens, or custom metadata are required.
• Added support for request body, allowing validation of endpoints that require POST requests or structured payloads.
• Updated internal logic to evaluate certificate expiry and generate alerts based on predefined thresholds.
• Aligned alerting behaviour with existing monitoring patterns for consistency.

Evidence:

The following alert was created successfully through pipeline, and running:

Test implementation:

Test execution:

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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