Add WriteThroughput tracking for async code path

[zoewangg]

Motivation and Context

This PR adds WRITE_THROUGHPUT metric support for the async client, completing the feature started in the sync client implementation. The
WRITE_THROUGHPUT metric measures the rate at which request body bytes are uploaded to the service (bytes/sec).

Modifications

• Added BytesWrittenTrackingPublisher to wrap async request body and track bytes/timing (mirrors sync BytesWrittenTrackingInputStream)
• Created RequestBodyMetrics container class to consolidate 3 execution attributes into 1, improving code organization
• Moved request body metrics attribute from SdkInternalExecutionAttribute (protected API) to InternalCoreExecutionAttribute (internal API)
• Updated async pipeline stages to initialize metrics and report WRITE_THROUGHPUT
• Refactored sync implementation to use the new RequestBodyMetrics container

Testing

• Unit tests for BytesWrittenTrackingPublisher
• Reactive Streams TCK compliance tests
• Functional tests with WireMock for both sync and async clients
• S3 integration tests for sync and async putObject
• JMH benchmark validated negligible overhead (<0.015% of network I/O time)

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.
• My change is to implement 1.11 parity feature and I have updated [LaunchChangelog](https://github.com/aws/aws-sdk-java-v2/blob/master/
  docs/LaunchChangelog.md)

License

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

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