routing: Use NUMS point for blinded paths

To be able to do MPP payment to multiple blinded routes we need
to add a constant dummy hop as a final hop to every blined path.
This is used when sending or querying a blinded path, to let the
pathfinder be able to send MPP payments over different blinded
routes. For this dummy final hop we use a NUMS key so that we
are sure no other node can use this blinded pubkey either in a
normal or blinded route.
Moreover this helps us handling the mission control data for
blinded paths correctly because we always consider the blinded
pubkey pairs which are registered with mission control when
a payment to a blinded path fails.

routing/pathfind_test.go

@@ -3284,9 +3284,7 @@ func TestBlindedRouteConstruction(t *testing.T) {
 	// that make up the graph we'll give to route construction. The hints
 	// map is keyed by source node, so we can retrieve our blinded edges
 	// accordingly.
-	blindedEdges, err := blindedPayment.toRouteHints(
-		fn.None[*btcec.PublicKey](),
-	)
+	blindedEdges, err := blindedPayment.toRouteHints()
 	require.NoError(t, err)
 
 	carolDaveEdge := blindedEdges[carolVertex][0]