FS backup Kopia uploader -- fallback to full backup if previous snapshot is not found

[Lyndon-Li]

For FS backup, a repo snapshot ID from the previous backup is passed to Kopia uploader as the parent of the current backup, so as to run the current backup incrementally.
Then Kopia uploader tries to get the snapshot from the repo, if the snapshot is not found, the backup will fail. Ideally, if the previous snapshot is not found, it should fallback to a full backup.

Error logs are like below:
time="2023-11-06T04:00:56Z" level=error msg="Error backing up item" backup=velero/backup-xxxxx error="pod volume backup failed: data path backup failed: Failed to run kopia backup: Failed to load previous snapshot 889cc6da7944acb1326a71a2ecdbed1c from kopia: snapshot not found" error.file="/go/src/github.com/vmware-tanzu/velero/pkg/podvolume/backupper.go:324" error.function="github.com/vmware-tanzu/velero/pkg/podvolume.(*backupper).BackupPodVolumes" logSource="pkg/backup/backup.go:448"

Only FS backup is affected, data mover backup doesn't have this problem.