Only create one delegation set

[lionello]

Description

The fix is to not keep creating new delegation sets, but only create when necessary. There's two situations:

• Hosted zone already exists. In this case we try to create the delegation set from the zone, which fails if it's "already reusable", in which case we get it from the zone directly;
• No hosted zone. We find an existing delegation set, and create one if none was found. Note that on subsequent deployments this will revert to the first case, ie. hosted zone already exists and we'll end up with the same delegation set that we created in the first place!

I've added tests that work both as unit tests (using R53 mock) and integration test!

Linked Issues

Fixes #946

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary

[jordanstephens]

Thank you for the refactor, it's much easier to follow! I still have some questions about how this works, but it seems reasonable given my limited understanding.