iceberg: s3tables region limit dyncfg

[DAlperin]

Use a dyncfg to decide whether to enforce the region check

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[def-]

What's the motivation? Why is it always off in CI?

[DAlperin]

@def- difference in behavior between cloud and self managed. Off by default because I don't want it clashing with minios made up regions in CI, but there is an explicit test covering it in test/aws