FIX: Make sure S3 object headers are preserved on copy (PR #14302)

When copying an existing upload stub temporary object on S3 to its final destination we were not copying across its additional headers such as content-disposition and cache-control, which led to issues like attachments not downloading with their original filename when clicking the download links in posts.

This is because the metadata_directive = REPLACE option was not being passed to object.copy_from(), so only the source object’s headers were being used. Added an option for apply_metadata_to_destination to apply this option conditionally, because we may not always want to replace this metadata, but we definitely do when copying a temporary upload.

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