Checksum GitLab managed object storage replication

This is a follow on from discussions on object storage verification in this issue #13845 (closed). We decided to take a different approach based on file size checks. Capture the original proposal here as a potential future improvement.

Proposal

• Add a setting e.g. Perform full checksum of object stored files
• When the setting is enabled, then do an actual SHA256 checksum instead of getting the file size.