routing+channeldb: add AddProof method

Originally we adding the edge without proof in order to able to use it
for payments path constrcution. This method will allow us to populate
the announcement proof after the exchange of the half proofs and
constrcutrion of full proof finished.

discovery/service_test.go

@@ -73,6 +73,11 @@ func (r *mockGraphSource) CurrentBlockHeight() (uint32, error) {
 	return r.bestHeight, nil
 }
 
+func (r *mockGraphSource) AddProof(chanID lnwire.ShortChannelID,
+	proof *channeldb.ChannelAuthProof) error {
+	return nil
+}
+
 func (r *mockGraphSource) ForEachNode(func(node *channeldb.LightningNode) error) error {
 	return nil
 }