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/payment_session_source.go

@@ -95,9 +95,9 @@ func (m *SessionSource) NewPaymentSessionEmpty() PaymentSession {
 // RouteHintsToEdges converts a list of invoice route hints to an edge map that
 // can be passed into pathfinding.
 func RouteHintsToEdges(routeHints [][]zpay32.HopHint, target route.Vertex) (
-	map[route.Vertex][]*models.CachedEdgePolicy, error) {
+	map[route.Vertex][]AdditionalEdge, error) {
 
-	edges := make(map[route.Vertex][]*models.CachedEdgePolicy)
+	edges := make(map[route.Vertex][]AdditionalEdge)
 
 	// Traverse through all of the available hop hints and include them in
 	// our edges map, indexed by the public key of the channel's starting
@@ -127,7 +127,7 @@ func RouteHintsToEdges(routeHints [][]zpay32.HopHint, target route.Vertex) (
 			// Finally, create the channel edge from the hop hint
 			// and add it to list of edges corresponding to the node
 			// at the start of the channel.
-			edge := &models.CachedEdgePolicy{
+			edgePolicy := &models.CachedEdgePolicy{
 				ToNodePubKey: func() route.Vertex {
 					return endNode.PubKeyBytes
 				},
@@ -142,6 +142,10 @@ func RouteHintsToEdges(routeHints [][]zpay32.HopHint, target route.Vertex) (
 				TimeLockDelta: hopHint.CLTVExpiryDelta,
 			}
 
+			edge := &PrivateEdge{
+				policy: edgePolicy,
+			}
+
 			v := route.NewVertex(hopHint.NodeID)
 			edges[v] = append(edges[v], edge)
 		}