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.

itest/lnd_route_blinding_test.go

@@ -676,7 +676,7 @@ func (b *blindedForwardTest) createBlindedRoute(hops []*forwardingEdge,
 
 		// Encode the route's blinded data and include it in the
 		// blinded hop.
-		payload := record.NewBlindedRouteData(
+		payload := record.NewNonFinalBlindedRouteData(
 			scid, nil, *relayInfo, constraints, nil,
 		)
 		payloadBytes, err := record.EncodeBlindedRouteData(payload)
@@ -739,7 +739,7 @@ func (b *blindedForwardTest) createBlindedRoute(hops []*forwardingEdge,
 		// node ID here so that it _looks like_ a valid
 		// forwarding hop (though in reality it's the last
 		// hop).
-		record.NewBlindedRouteData(
+		record.NewNonFinalBlindedRouteData(
 			lnwire.NewShortChanIDFromInt(100), nil,
 			record.PaymentRelayInfo{}, nil, nil,
 		),