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.

discovery/gossiper.go

@@ -171,6 +171,10 @@ type Config struct {
 	// order to be included in the LN graph.
 	Graph graph.ChannelGraphSource
 
+	// ChainIO represents an abstraction over a source that can query the
+	// blockchain.
+	ChainIO lnwallet.BlockChainIO
+
 	// ChanSeries is an interfaces that provides access to a time series
 	// view of the current known channel graph. Each GossipSyncer enabled
 	// peer will utilize this in order to create and respond to channel
@@ -1930,6 +1934,13 @@ func (d *AuthenticatedGossiper) processRejectedEdge(
 	return announcements, nil
 }
 
+// fetchPKScript fetches the output script for the given SCID.
+func (d *AuthenticatedGossiper) fetchPKScript(chanID *lnwire.ShortChannelID) (
+	[]byte, error) {
+
+	return lnwallet.FetchPKScriptWithQuit(d.cfg.ChainIO, chanID, d.quit)
+}
+
 // addNode processes the given node announcement, and adds it to our channel
 // graph.
 func (d *AuthenticatedGossiper) addNode(msg *lnwire.NodeAnnouncement,