Add validation for commits/PRs to main/staging

[jason-ng3]

1. Adds a GitHub Actions workflow, for which any push or pull request to staging or main with changed files in the templates directory will invoke a validation script, passing in all files for the affected vendor-product directory or directories.

2. Adds the validation script which:

   • Checks that all required files are present.
   • Validates structure of and type checks asset YAML and manifest YAML using Pydantic library.
   • Validates cross-file references.
   • If validation does not pass, the event is rejected and logs print a list of the affected files and their respective errors.

3. Tested script locally and then tested GitHub Actions with happy/error paths using pushes to this feature branch and pull requests from a test branch to this feature branch.