multi: announce Keysend feature bit

In this commit, we add the keysend feature bit to our NodeAnnouncement
if the accept-keysend option is set.

feature/default_sets.go

@@ -64,6 +64,9 @@ var defaultSetDesc = setDesc{
 		SetInit:    {}, // I
 		SetNodeAnn: {}, // N
 	},
+	lnwire.KeysendOptional: {
+		SetNodeAnn: {}, // N
+	},
 	lnwire.ScriptEnforcedLeaseOptional: {
 		SetInit:    {}, // I
 		SetNodeAnn: {}, // N