Add support for multiple virtual_storages in praefect

Summary

Initial support was only for a default storage, with N number of Gitaly pods.

We need to add support for an arbitrary list of virtual_storage entries, with the appropriate Gitaly instances behind them.

Upgrading existing deployments to 13.8 and later

(Edited to add this section as part of working a customer ticket (internal link))

The 13.8 release notes explain how to handle the naming changes associated with this.

the StatefulSet name for Gitaly will now include the virtual storage name. Because of this name change, any existing Praefect-managed Gitaly StatefulSet names (and, therefore, their associated PersistentVolumeClaims) will change as well, leading to repository data appearing to be lost. In this case, repository data can be restored by following the managing persistent volumes documentation, which provides guidance on reconnecting existing PersistentVolumeClaims to previous PersistentVolumes.