Github action to validate yml schemas

[jfaurskov]

Overview/Summary

Add code to validate alert yaml schema on PR. Task is described in https://dev.azure.com/CSUSolEng/Guidance%20-%20Observability/_workitems/edit/37696

This PR fixes/adds/changes/removes

1. Adds Github action .github/workflows/validate-yml-schema.yml, which will trigger on PR.
2. Adds .github/yml-schemas directory containing the default yml schemas for the various alert types.

Breaking Changes

May cause previously accepted yml files to be rejected if changes are made since they are not meeting requirements.

As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues or ADO Work Items (Internal Only)
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Ensured PR tests are passing
• Updated relevant and associated documentation (e.g. Contribution Guide, Docs etc.)

[judyer28]

@jfaurskov, please fix errors raised by lint checks.

[judyer28]

LGTM. Thanks Jan!