Chore: improved sample input vars

[aaronsteers]

This PR suggests two relatively small non-breaking changes to the sample project:

1. Mark secrets with "sensitive = true". This informs terraform to treat them as sensitive, for instance: preventing their echoing during user entry and redacting from plan and from log files.
2. Add prompts (descriptions) to each variable input, which just makes those inputs more obvious to the user if typing in via the terraform CLI built-in Terraform prompts.

Screenshots:

Context:

• I implemented these changes as I was testing (Draft) Chore: Add Devcontainer config to enable testing in GitHub Codespaces #5, and I cherry picked the initial commits from that PR

[aaronsteers]

CI tests passed. 🚀

This is ready for review.