Add expectContinueThresholdInBytes config to S3

[alextwoods]

Motivation and Context

The Expect: 100-continue header allows a client to check whether the server will accept a request before sending the body. While this saves bandwidth on rejected requests, it adds a full round-trip of latency on every successful request. Benchmark data shows this overhead can increase latency by up to 30% for small uploads (under 1 MB) on public internet.

Currently the SDK unconditionally adds the header to all PutObject and UploadPart requests (when expectContinueEnabled is true). This change introduces a configurable content-length threshold so the header is skipped for small payloads where the cost outweighs the benefit.

This is consistent with other AWS SDKs — JavaScript V3 and Kotlin both use a content-length threshold for this header.

Related to #6774 and #6828

Modifications

• Added expectContinueThresholdInBytes configuration with default of 1MB.

Testing

new + existing unit tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license