Support object storage bucket prefixes

In gitlab-org/gitlab!91307 (merged) we are adding support for object storage bucket prefixes. This allows administrators to install GitLab using fewer object storage buckets: instead of separate buckets for uploads, artifacts, lfs etc. it will be possible to have one bucket my-gitlab-bucket and configure GitLab to store data in my-gitlab-bucket/uploads, my-gitlab-bucket/artifacts, my-gitlab-bucket/lfs, etc.

Currently our documentation discourages using a single buckets because there is some (LINK?) problem with CNG backups: https://docs.gitlab.com/ee/administration/object_storage.html#use-separate-buckets. Looking at https://gitlab.com/gitlab-org/build/CNG/-/blob/904b26e6582b1349f8f8029264163544ee467e2c/gitlab-toolbox/scripts/lib/object_storage_backup.rb#L29-40 there may not be a problem because most rsync style tools (s3cmd sync etc) support prefixes. But we can't be sure until we try this.