routing/route+tlv: add new TLV-EOB awareness to Hop+Route

In this commit, we extend the Hop struct to carry an arbitrary set of
TLV values, and add a new field that allows us to distinguish between
the modern and legacy TLV payload.

We add a new `PackPayload` method that will be used to encode the
combined required routing TLV fields along any set of TLV fields that
were specified as part of path finding.

Finally, the `ToSphinxPath` has been extended to be able to recognize if
a hop needs the modern, or legacy payload.

routing/control_tower_test.go

@@ -142,6 +142,7 @@ func TestControlTowerSubscribeSuccess(t *testing.T) {
 		if result.Preimage != preimg {
 			t.Fatal("unexpected preimage")
 		}
+
 		if !reflect.DeepEqual(result.Route, &attempt.Route) {
 			t.Fatal("unexpected route")
 		}