Implement a wrokflow to ensure `opensaas-sh` diffs are up to date

Implement a workflow which would check if generating a diff makes changes to the working directory. If it does it fails the PR. 
This it to make sure our diffs are up to date with the template changes.

Read more here: https://github.com/wasp-lang/open-saas/pull/505#discussion_r2509900453