multi: add custom message overrides to protocol config options

lncfg/protocol_experimental_off.go

@@ -7,3 +7,9 @@ package lncfg
 // features that also require a build-tag to activate.
 type ExperimentalProtocol struct {
 }
+
+// CustomMessageOverrides returns the set of protocol messages that we override
+// to allow custom handling.
+func (p ExperimentalProtocol) CustomMessageOverrides() []uint16 {
+	return nil
+}

lncfg/protocol_experimental_on.go

@@ -5,5 +5,14 @@ package lncfg
 
 // ExperimentalProtocol is a sub-config that houses any experimental protocol
 // features that also require a build-tag to activate.
+//
+//nolint:lll
 type ExperimentalProtocol struct {
+	CustomMessage []uint16 `long:"custom-message" description:"allows the custom message apis to send and report messages with the protocol number provided that fall outside of the custom message number range."`
+}
+
+// CustomMessageOverrides returns the set of protocol messages that we override
+// to allow custom handling.
+func (p ExperimentalProtocol) CustomMessageOverrides() []uint16 {
+	return p.CustomMessage
 }