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`.

htlcswitch/payment_result_test.go

@@ -38,7 +38,6 @@ func TestNetworkResultSerialization(t *testing.T) {
 		ChanID:          chanID,
 		ID:              2,
 		PaymentPreimage: preimage,
-		ExtraData:       make([]byte, 0),
 	}
 
 	fail := &lnwire.UpdateFailHTLC{