fix(s3_bucket_downloader): add redownload mode for missing files

[sangshuduo]

• Introduced a new redownload mode to check existing files and only download missing ones.
• Added a command-line argument to enable this feature.
• Updated progress reporting to reflect the number of existing and missing files.

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[mentatbot]

I've reviewed this PR and the implementation looks good! The redownload mode is a useful feature that will help save bandwidth and time by only downloading missing files.

Some notes on the implementation:

• The command-line argument is properly added with good documentation
• The file existence checking logic is clear and straightforward
• The progress reporting is updated appropriately to show existing vs. missing file counts
• The early return when no files need downloading is a good optimization

The code maintains the existing behavior when redownload mode is not enabled, which is great.

One small suggestion: the thread progress message changed from showing detailed counts to a simpler "by Thread X" message. This seems less informative than before, but it's a minor UI detail.

Overall, this is a well-implemented feature that adds useful functionality. Approving!