Add support for object storage bucket prefixes

Part of gitlab-com/gl-infra/scalability#1792 (closed).

In #365810 (comment 1007343035) we discovered that in some cases, it was possible to have GitLab use one object storage bucket by putting a / in each remote_directory: my-gitlab-objects/uploads, my-gitlab-objects/artifacts, etc. The deprecation of background uploads in GitLab 15.0 has forced some administrators to change their configuration to a case where these path prefixes do not work.

This MR adds explicit support for path prefixes. This is beneficial for installations that were already using them "unofficially", and it will make it possible for new installations to use a single bucket from the start.