Add support for Content-MD5 or x-amz-checksum-* headers for Object Lock compatibility

[pmng-ncd]

Hi and thanks for your work on this project!

I'm currently using pmoxs3backuproxy to push backups to an S3-compatible backend (e.g., Backblaze B2 / Wasabi / AWS S3) with Object Lock enabled.

Unfortunately, any PUT Object request to a bucket with Object Lock requires either a Content-MD5 header or a x-amz-checksum-* header (e.g., x-amz-checksum-sha256). Without this, the request fails with:

Content-MD5 OR x-amz-checksum- HTTP header is required for Put Object requests with Object Lock parameters

Feature request:

Would it be possible to add automatic support for setting one of the required headers (e.g., Content-MD5) when pushing objects to S3?

Ideally, this would be:

• Optional (via a flag like --require-content-md5 or env var REQUIRE_CONTENT_MD5=true)
• Automatically applied when the proxy detects a bucket with Object Lock, or forced via config

This would make the proxy fully compatible with S3 buckets that have Object Lock enabled, which is often required for backup compliance.

Let me know if I can help to test this feature!

Thanks again 🙏