Add SSE-C key configuration option for Amazon S3 remote backups

What does this MR do?

Amazon S3 supports three styles of server-side encryption: KMS managed keys, S3 managed keys and customer-provided keys. Currently only S3 managed keys are supported.

This MR adds the capability to also use customer-provided keys by exposing the encryption_key attribute that's already supported in Fog.

What are the relevant issue numbers?

None

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer