discovery+lnwallet: add fetchPKScript helper to gossiper

This commit makes an lnwallet.BlockChainIO available to the gossiper and
uses it to construct a helper that can be used to fetch the pk script
for a given SCID. This will be used for channel announcement
verification in an upcoming commit.

server.go

@@ -1042,6 +1042,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.authGossiper = discovery.New(discovery.Config{
 		Graph:                 s.graphBuilder,
+		ChainIO:               s.cc.ChainIO,
 		Notifier:              s.cc.ChainNotifier,
 		ChainHash:             *s.cfg.ActiveNetParams.GenesisHash,
 		Broadcast:             s.BroadcastMessage,