routing: add new interface AdditionalEdge.

A new interface AdditionalEdge is introduced which allows us to
enhance the CachedEdgePolicy with additional information. We
currently only need a PayloadSize function which is then used to
determine the exact payload size when building a route to adhere
to the sphinx package size limit (BOLT04).

routing/mocks.go

@@ -0,0 +1,32 @@
+package routing
+
+import (
+	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/stretchr/testify/mock"
+)
+
+// mockAdditionalEdge is a mock of the AdditionalEdge interface.
+type mockAdditionalEdge struct{ mock.Mock }
+
+// IntermediatePayloadSize returns the sphinx payload size defined in BOLT04 if
+// this edge were to be included in a route.
+func (m *mockAdditionalEdge) IntermediatePayloadSize(amount lnwire.MilliSatoshi,
+	expiry uint32, legacy bool, channelID uint64) uint64 {
+
+	args := m.Called(amount, expiry, legacy, channelID)
+
+	return args.Get(0).(uint64)
+}
+
+// EdgePolicy return the policy of the mockAdditionalEdge.
+func (m *mockAdditionalEdge) EdgePolicy() *models.CachedEdgePolicy {
+	args := m.Called()
+
+	edgePolicy := args.Get(0)
+	if edgePolicy == nil {
+		return nil
+	}
+
+	return edgePolicy.(*models.CachedEdgePolicy)
+}