add: waitForDomain func to wait for a domain to be resolvable before pinging lets-encrypt

[iwilltry42]

Tiny helper function so we don't bother LE with unnecessary requests, just because the domain hasn't been propagated yet.

Checklist

• [ ] The title of this PR would make a good line in Acorn's Release Note's Changelog
• [ ] 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
• [ ] All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• [ ] 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
• [ ] Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
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