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
gencryption 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`).

This also refactors the object storage config parsing.

This requires
https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/537 to
work.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/22200