Calculate SHA-256 Checksum for Exportable Backup Files

It is just like any other transfer over the internet; the downloadable backups are prone to certain errors and file corruptions. These events are largely unlikely but still possible, so Capella ought to provide its customers with certain guarantees to ensure the integrity of downloadable backups.

The plan is to:

1. Provide a checksum value using SHA-256 (Secure Hash Algorithm) to customers so they can cross-check the integrity of their backup exports.
2. Calculate the hash value on the fly locally as dp-backup uploads the zipped data chunks to S3 to avoid additional computing costs.

The entire plan is available on https://couchbasecloud.atlassian.net/wiki/spaces/BI/pages/2200535090/Downloadable+Backup+Integrity.