[CIVIS-8846] ENH validate a workflow YAML definition

[jacksonlee-civis]

This pull request adds the function civis.workflows.validate_workflow_yaml to validate a Civis Platform workflow YAML definition.

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• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
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