Iceberg 1.10.0: Unexpected S3 checksum validation despite s3.checksum-enabled=false

### Apache Iceberg version

1.10.0 (latest release)

### Query engine

None

### Please describe the bug 🐞

Starting in Iceberg 1.10.0, checksum validation began to be enforced for S3 PutObject operations. This happens even when the Iceberg config `s3.checksum-enabled` is set to its default of false.

At first glance this looks like an Iceberg bug, but it actually stems from an AWS SDK policy change: the AWS client’s `AWS_REQUEST_CHECKSUM_CALCULATION` default was changed to `WHEN_SUPPORTED`. With that default, the client automatically performs checksum calculation whenever the service supports it, even if the caller didn’t explicitly enable checksums.
(https://docs.aws.amazon.com/sdkref/latest/guide/feature-dataintegrity.html)

Proposal:
When initializing the S3 client in Iceberg, set `AWS_REQUEST_CHECKSUM_CALCULATION` to `WHEN_REQUIRED` so that checksums are only calculated when strictly required. Then allow users to control checksum usage via Iceberg’s `s3.checksum-enabled` setting.

This way:
- Users who want checksums can enable them explicitly via s3.checksum-enabled.
- Users who keep the default (false) won’t incur checksum calculation/validation unexpectedly after upgrading to 1.10.0.

What do you think about this approach?

### Willingness to contribute

- [x] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time