S3 Directory Support

[jterapin]

Adds directory upload/download to Transfer Manager.

upload_directory

• Upload all files from a local directory to S3
• Optional recursive traversal of subdirectories
• Symlink handling with circular reference detection
• S3 key prefix support
• Filter callback to selectively upload files
• Request callback to modify upload parameters per file
• Progress callback for transfer monitoring
• Configurable failure handling (fail-fast or continue on error)

download_directory

• Download all objects from an S3 bucket/prefix to a local directory
• S3 prefix stripping for clean local paths
• Path traversal detection
• Filter callback to selectively download objects
• Request callback to modify download parameters per object
• Progress callback for transfer monitoring
• Configurable failure handling (fail-fast or continue on error)
• Automatic directory creation for nested structures

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1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/version-3/CONTRIBUTING.md

Thank you for your contribution!

[github-actions]

Detected 1 possible performance regressions:

• aws-sdk-s3.gem_size_kb - z-score regression: 549.5 -> 555.0. Z-score: Infinity

[alextwoods]

LGTM - just minor comments.