fix: encrypt checksums in metadata (#15620)

cmd/storage-datatypes.go

@@ -231,8 +231,7 @@ type FileInfo struct {
 	DiskMTime time.Time `msg:"dmt"`
 
 	// Combined checksum when object was uploaded.
-	// Format is type:base64(checksum).
-	Checksum map[string]string `msg:"cs,allownil"`
+	Checksum []byte `msg:"cs,allownil"`
 }
 
 // Equals checks if fi(FileInfo) matches ofi(FileInfo)