Combine identically-named TXT records

[jsf9k]

🗣 Description

This pull request combines TXT records with identical name attributes.

💭 Motivation and Context

This corrects a bug in #38 by @cablej that went undetected by me, our CI pipeline, and the human reviewers. I only noticed it when I attempted to apply the changes via Terraform after merging the PR. Terraform cannot create two TXT records with the same name attribute; they must be combined into a single record.

🧪 Testing

These changes have already been applied to COOL production.

✅ Checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.