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.go

@@ -1954,7 +1954,7 @@ type RouteRequest struct {
 
 // RouteHints is an alias type for a set of route hints, with the source node
 // as the map's key and the details of the hint(s) in the edge policy.
-type RouteHints map[route.Vertex][]*models.CachedEdgePolicy
+type RouteHints map[route.Vertex][]AdditionalEdge
 
 // NewRouteRequest produces a new route request for a regular payment or one
 // to a blinded route, validating that the target, routeHints and finalExpiry