For CI workflow templates, add mechanism for scanning for secrets and give user an opportunity to create them if they're not set in the provider source

For templates tagged "github-workflow", we should scan the source file for secrets that it depends on. Attempt to use `gh` CLI tool to see if the secrets are set in GitHub repo (or org), just to verify that all of the secrets used by the workflow exist. If they don't exist, prompt user to set the secrets.