Unified Backups: Investigate what is required to support kubernetes and large reference architectures

Context

With the GitLab Unified Backups we are initially covering the use-cases under the Simple Backup Procedure.

When that is released, we need to look at the more advanced and scaled architectures. Because existing solution is a patchwork of workarounds, we should evaluate previous decisions first before just enabling/reimplementing them.

Some initial guidance:

• An scaled application likely runs on Object Storage (so we need to support that)
• A backup taken from a local storage installation should behave the same as one taken from an object storage installation
• Any backup should be possible to restore on a local storage installation or object storage installation
• A backup should also be interchangeable no matter the installation type: GDK/Source, Omnibus, Docker and Kubernetes
  • It's expected that in case we bundle installation specific configuration (like /etc/gitlab/gitlab.br that would not be re-usable on other installation methods)

This needs to address concerns of at least Reference architecture: up to 3,000 users