config: add new legacy protocol build tag protected sub-config for legacy onion

In this commit, we add a new build tag protected sub-config for legacy
protocol features. The goal of this addition is to be able to default to
new feature within lnd, but expose hooks at the config level to allow
integration tests to force the old behavior to ensure that we're able to
support both the old+new versions.

config.go

@@ -328,6 +328,8 @@ type config struct {
 	WtClient *lncfg.WtClient `group:"wtclient" namespace:"wtclient"`
 
 	Watchtower *lncfg.Watchtower `group:"watchtower" namespace:"watchtower"`
+
+	LegacyProtocol *lncfg.LegacyProtocol `group:"legacyprotocol" namespace:"legacyprotocol"`
 }
 
 // loadConfig initializes and parses the config using a config file and command