multi: use new AdditionalEdge interface.

In the previous commit the AdditionalEdge interface was introduced
with both of its implementations `BlindedEdge` and `PrivateEdge`.
In both cases where we append a route either by a blinded route
section or private route hints we now use these new types. In
addition the `PayloadSizeFunc` type is introduced in the
`unifiedEdge` struct. This is necessary to have the payload size
function at hand when searching for a route hence not overshooting
the max sphinx package size of 1300 bytes.

routing/router_test.go

@@ -3954,7 +3954,7 @@ func TestNewRouteRequest(t *testing.T) {
 			name:           "hints and blinded",
 			blindedPayment: blindedMultiHop,
 			routeHints: make(
-				map[route.Vertex][]*models.CachedEdgePolicy,
+				map[route.Vertex][]AdditionalEdge,
 			),
 			err: ErrHintsAndBlinded,
 		},