Integrate with Velero Download Server

[kaovilai]

Depending on how vmware-tanzu/velero#6167 gets designed and implemented, it might make sense to rework this plugin's file server architecture around it.

[reasonerjt]

We need a solution for the plugin to have good support on DownloadRequest CR by velero.
Technically, it may NOT depend on the implementation of vmware-tanzu/velero#6167, as IMO it is still an immature idea.