lnwire: add custom records field to type UpdateFulfillHtlc

- Introduce the field `CustomRecords` to the type `UpdateFulfillHtlc`.
- Encode and decode the new field into the `ExtraData` field of the
`update_fulfill_htlc` wire message.
- Empty `ExtraData` field is set to `nil`.

peer/brontide.go

@@ -2202,8 +2202,9 @@ func messageSummary(msg lnwire.Message) string {
 			msg.ID, msg.Reason)
 
 	case *lnwire.UpdateFulfillHTLC:
-		return fmt.Sprintf("chan_id=%v, id=%v, pre_image=%x",
-			msg.ChanID, msg.ID, msg.PaymentPreimage[:])
+		return fmt.Sprintf("chan_id=%v, id=%v, pre_image=%x, "+
+			"custom_records=%v", msg.ChanID, msg.ID,
+			msg.PaymentPreimage[:], msg.CustomRecords)
 
 	case *lnwire.CommitSig:
 		return fmt.Sprintf("chan_id=%v, num_htlcs=%v", msg.ChanID,