Object Store (AWS): Support dynamically resolving S3 bucket region

[mr-brobot]

Which issue does this PR close?

Closes #3213.

Rationale for this change

See #3213.

What changes are included in this PR?

Are there any user-facing changes?

Added a new aws::resolve_bucket_region function that uses the HeadBucket API to get the bucket region.

[tustvold]

Really nice find that it returns the header even on permission error 👍

Given this doesn't use anything on the builder, what do you think about making this a free function that takes a bucket name and &ClientOptions and returns the region? This would keep the builder signature slightly more consistent? Maybe something like resolve_bucket_region?

[mr-brobot]

@tustvold I like that suggestion. Implemented as aws::resolve_bucket_region. ✅

I'd like your feedback on the tests too... They hit the S3 HeadBucket API using some bucket names that work today, but that might not be true forever (someone could create a bucket named "please-dont-exist").

This was to show it working in PR, but maybe better to convert to unit tests by creating a private head_bucket function with different implementations for test/live?

[tustvold]

but that might not be true forever

I think this is fine, we can always update the tests should this change. I don't think it is worth the added complexity to mock it out, you would end up with more test logic than actual logic 😆

[tustvold]

I took the liberty of pushing 5c5fc07 to clean the error handling up a little, prior to getting this in

[tustvold]

Thank you for working on this