multi: make MsgRouter available in the ImplementationCfg

With this commit, we allow the `MsgRouter` to be available in the
`ImplementationCfg`. With this, programs outside of lnd itself are able
to now hook into the message processing flow to direct handle custom
messages, and even normal wire messages.

peer/brontide.go

@@ -387,6 +387,11 @@ type Config struct {
 	// This value will be passed to created links.
 	MaxFeeExposure lnwire.MilliSatoshi
 
+	// MsgRouter is an optional instance of the main message router that
+	// the peer will use. If None, then a new default version will be used
+	// in place.
+	MsgRouter fn.Option[msgmux.Router]
+
 	// Quit is the server's quit channel. If this is closed, we halt operation.
 	Quit chan struct{}
 }
@@ -542,6 +547,12 @@ var _ lnpeer.Peer = (*Brontide)(nil)
 func NewBrontide(cfg Config) *Brontide {
 	logPrefix := fmt.Sprintf("Peer(%x):", cfg.PubKeyBytes)
 
+	// We'll either use the msg router instance passed in, or create a new
+	// blank instance.
+	msgRouter := cfg.MsgRouter.Alt(fn.Some[msgmux.Router](
+		msgmux.NewMultiMsgRouter(),
+	))
+
 	p := &Brontide{
 		cfg:           cfg,
 		activeSignal:  make(chan struct{}),
@@ -564,9 +575,7 @@ func NewBrontide(cfg Config) *Brontide {
 		startReady:         make(chan struct{}),
 		quit:               make(chan struct{}),
 		log:                build.NewPrefixLog(logPrefix, peerLog),
-		msgRouter: fn.Some[msgmux.Router](
-			msgmux.NewMultiMsgRouter(),
-		),
+		msgRouter:          msgRouter,
 	}
 
 	if cfg.Conn != nil && cfg.Conn.RemoteAddr() != nil {