Prior to this change, uploads to AWS S3 were only encrypted on the
server if a default encryption were specified on the bucket.

With this change, admins can now configure the encryption and the AWS
Key Management Service (KMS) key ID in GitLab Rails, and the
configuration will be used in uploads. Bucket policies to enforce
encryption can now be used since Workhorse sends the required headers
(`x-amz-server-side-encryption` and
`x-amz-server-side-encryption-aws-kms-key-id`). The bucket policy cannot
be enforced with default encryption, since that is applied after the
check.

This requires the changes in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38240 to work.

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/22200