enhance: allow dots in env vars

[iwilltry42]

Before, one couldn't have dots . in env var keys, but services like OpenSearch use them quite a bit. We're using . as hierarchy separators during interpolation, which caused dotted env var keys to be ignored silently. Now we're escaping dotted keys first and restoring the original later.

Checklist

• [x] 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
• [x] Commits follow contributing guidance
• [x] 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
• [ ] PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)