Add mani-diffy

[bwhaley]

This program walks a hierarchy of Argo CD Application templates, renders Kubernetes manifests from the input templates, and posts the rendered files back for the user to review and validate.

It is designed to be called from a CI job within a pull request, enabling the author to update templates and see the resulting manifests directly within the pull request before the changes are applied to the Kubernetes cluster.

https://github.com/1debit/mani-diffy