record+htlcswitch: convert BlindedRouteData fields to optional

For the final hop in a blinded route, the SCID and RelayInfo fields will
_not_ be set. So these fields need to be converted to optional records.

The existing BlindedRouteData constructor is also renamed to
`NewNonFinalBlindedRouteData` in preparation for a
`NewFinalBlindedRouteData` constructor which will be used to construct
the blinded data for the final hop which will contain a much smaller set
of data. The SCID and RelayInfo parameters of the constructor are left
as non-pointers in order to force the caller to set them in the case
that the constructor is called for non-final nodes. The other option
would be to create a single constructor where all parameters are
optional but I think this makes it easier for the caller to make a
mistake.

htlcswitch/hop/iterator_test.go

@@ -186,7 +186,7 @@ func TestDecryptAndValidateFwdInfo(t *testing.T) {
 
 	// Encode valid blinding data that we'll fake decrypting for our test.
 	maxCltv := 1000
-	blindedData := record.NewBlindedRouteData(
+	blindedData := record.NewNonFinalBlindedRouteData(
 		lnwire.NewShortChanIDFromInt(1500), nil,
 		record.PaymentRelayInfo{
 			CltvExpiryDelta: 10,