multi/refactor: separate methods for get and set node announcement

In preparation for a more complex function signature for set node
announcement, separate get and set so that readonly callers don't need
to handle the extra arguments.

subrpcserver_config.go

@@ -118,7 +118,7 @@ func (s *subRPCServerConfigs) PopulateDependencies(cfg *Config,
 	tcpResolver lncfg.TCPResolver,
 	genInvoiceFeatures func() *lnwire.FeatureVector,
 	genAmpInvoiceFeatures func() *lnwire.FeatureVector,
-	getNodeAnnouncement func() (lnwire.NodeAnnouncement, error),
+	getNodeAnnouncement func() lnwire.NodeAnnouncement,
 	updateNodeAnnouncement func(modifiers ...netann.NodeAnnModifier) error,
 	parseAddr func(addr string) (net.Addr, error),
 	rpcLogger btclog.Logger,