storage/s3: We should respect `client.region` too

[Xuanwo]

Currently, we are loading the region from s3.region. However, client.region is used in the REST catalog and Java implementation. It would be better for us to also respect client.region.

[jdockerty]

I can do this 👍

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It would be better for us to also respect client.region.

If I understand rightly, the ask here to keep the s3.region within iceberg-rust and respect config.region as with the Java implementation?

Edit: I've put up a quick PR for this, feel free to suggest any changes if I've misunderstood the ask.