routing: extend path finding to be TLV-EOB aware, allow dest TLV records

In this commit, we extend the path finding to be able to recognize when
a node needs the new TLV format, or the legacy format based on the
feature bits they expose. We also extend the `LightningPayment` struct
to allow the caller to specify an arbitrary set of TLV records which can
be used for a number of use-cases including various variants of
spontaneous payments.

routing/payment_session.go

@@ -127,6 +127,7 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	sourceVertex := route.Vertex(ss.SelfNode.PubKeyBytes)
 	route, err := newRoute(
 		payment.Amount, sourceVertex, path, height, finalCltvDelta,
+		payment.FinalDestRecords,
 	)
 	if err != nil {
 		// TODO(roasbeef): return which edge/vertex didn't work