listobjectversions: Add shortcut for Veeam blocks (#10893)

Add shortcut for `APN/1.0 Veeam/1.0 Backup/10.0`

It requests unique blocks with a specific prefix. We skip 
scanning the parent directory for more objects matching the prefix.

cmd/metacache-server-sets.go

@@ -83,6 +83,10 @@ func (z *erasureServerSets) listPath(ctx context.Context, o listPathOptions) (en
 		o.ID = mustGetUUID()
 	}
 	o.BaseDir = baseDirFromPrefix(o.Prefix)
+	if o.singleObject {
+		// Override for single object.
+		o.BaseDir = o.Prefix
+	}
 
 	var cache metacache
 	// If we don't have a list id we must ask the server if it has a cache or create a new.