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.

lncfg/protocol_legacy_off.go

@@ -0,0 +1,16 @@
+// +build !dev
+
+package lncfg
+
+// LegacyProtocol is a struct that we use to be able to test backwards
+// compatibility of protocol additions, while defaulting to the latest within
+// lnd.
+type LegacyProtocol struct {
+}
+
+// LegacyOnion returns true if the old legacy onion format should be used when
+// we're an intermediate or final hop. This controls if we set the
+// TLVOnionPayloadOptional bit or not.
+func (l *LegacyProtocol) LegacyOnion() bool {
+	return false
+}