* s3: preserve checksums for copied multipart parts
* s3: return checksums from multipart copy
* s3: pin the upload's checksum algorithm on copy-part re-stream
* s3: note why UploadPartCopy uses the re-stream slow path
* s3: explain the TLS proxy in the multipart copy checksum test
* s3: cover nil and unknown-algorithm edge cases in copy checksum tests
* s3: cover all checksum algorithms in the multipart copy test
* s3: run all checksum integration tests, not just presigned
Chris LuandGitHub
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