fix: credential secrets: use defaults as defined in the Acornfile (#2411)

[g-linville]

for #2411

Now when the user is prompted to enter values for a credential secret, the default will be displayed to them, and if they enter nothing into the prompt, then the default will be used.

Checklist

• [x] The title of this PR would make a good line in Acorn's Release Note's Changelog
• [x] The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• [x] All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• [x] Commits follow contributing guidance
• [ ] Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• [x] Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
• [ ] PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)