feat: Added support for Discrimintor-based variants to readme utility

[AlexanderSehr]

Description

• Variants of a type are listed below the property with the discriminator and are recurisvely resolved just as classic parameter - just with the discriminator in the header
• Also, because it surfaced in this PR for two modules, I implemented a fix for the ReadMe test that previously (if failed) could under specific cirmustances also fail the GitHub Output. The circmustance was that the Set-ModuleReadMe script would throw an exception during the generation, returning the error that would hit the Pester-Output which in turn would pollute the test results the GitHub-Output function then failed to interpret and publish. The fix essentially catches this exception and fails the test with a proper error message, pointing to the actual cause of the error (e.g., a parameter not having a description category specified). See screenshots at the buttom for a before-after comparison.
• Finally, I implemented a fix for the parameter-resolving function used by out pester tests that was unable to resolve types using a discriminator

Required module fixes

• Updated the Virtual-Network-Gateway itself as its incorrect UDT(s) would make running the readme generation and passing the tests impossible. Changes include
  • Added missing property descriptions
  • Fixed an incorrect category for a parameter
  • Added a UDT for the NAT Rules child module
  • Updated tests
• Updated the Insights-Metric-Alert itself as its incorrect UDT(s) would make running the readme generation and passing the tests impossible. Changes include
  • Added missing property descriptions
  • Updated tests

Closes #4128

• TODO: Test with more recursive test templates
• TODO: Regenerate all readmes
• TODO: Run tests via workflows

Pipeline Reference

Pipeline
Modules without discriminator

Type of Change

• Update to CI Environment or utilities (Non-module affecting changes)
• Azure Verified Module updates:
  • Bugfix containing backwards-compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in version.json:
    • Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • The bug was found by the module author, and no one has opened an issue to report it yet.
  • Feature update backwards compatible feature updates, and I have bumped the MINOR version in version.json.
  • Breaking changes and I have bumped the MAJOR version in version.json.
  • Update to documentation

Appendix

Error before the fix

Error after the fix