Add S3 Express bucket metric tracking-v2

[vcjana]

Motivation and Context

Track S3 Express bucket credential usage in User-Agent metrics to provide visibility into S3 Express adoption and usage patterns.

Description

This PR adds tracking for S3 Express bucket credentials by embedding the S3ExpressBucket feature in credentials resolved through the S3 Express identity provider. The feature is reported as metric code J in the User-Agent header.

Testing

• Unit tests verify the feature is correctly embedded in S3 Express credentials
• Integration tests verify metric J appears in User-Agent for S3 Express bucket operations
• Integration tests verify metric J does NOT appear when S3 Express auth is disabled or for regular buckets

Checklist

• I have updated CHANGELOG.next.toml if I made changes to the smithy-rs codegen or runtime crates
• I have updated CHANGELOG.next.toml if I made changes to the AWS SDK, generated SDK code, or SDK runtime crates

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