multi: endpoints for onion messages

This commit creates the necessary endpoints for onion messages.
Specifically, it adds the following:

- `SendOnionMessage` endpoint to send onion messages.
- `SubscribeOnionMessages` endpoint to subscribe to incoming onion
  messages.

It uses the `msgmux` package to handle the onion messages.

peer/brontide.go

@@ -47,6 +47,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/msgmux"
 	"github.com/lightningnetwork/lnd/netann"
+	"github.com/lightningnetwork/lnd/onionmessage"
 	"github.com/lightningnetwork/lnd/pool"
 	"github.com/lightningnetwork/lnd/protofsm"
 	"github.com/lightningnetwork/lnd/queue"
@@ -463,6 +464,10 @@ type Config struct {
 	// related wire messages.
 	AuxChannelNegotiator fn.Option[lnwallet.AuxChannelNegotiator]
 
+	// OnionMessageServer is an instance of a message server that dispatches
+	// onion messages to subscribers.
+	OnionMessageServer *subscribe.Server
+
 	// ShouldFwdExpEndorsement is a closure that indicates whether
 	// experimental endorsement signals should be set.
 	ShouldFwdExpEndorsement func() bool
@@ -898,6 +903,21 @@ func (p *Brontide) Start() error {
 		return fmt.Errorf("unable to load channels: %w", err)
 	}
 
+	onionMessageEndpoint := onionmessage.NewOnionEndpoint(
+		p.cfg.OnionMessageServer,
+	)
+
+	// We register the onion message endpoint with the message router.
+	err = fn.MapOptionZ(p.msgRouter, func(r msgmux.Router) error {
+		_ = r.UnregisterEndpoint(onionMessageEndpoint.Name())
+
+		return r.RegisterEndpoint(onionMessageEndpoint)
+	})
+	if err != nil {
+		return fmt.Errorf("unable to register endpoint for onion "+
+			"messaging: %w", err)
+	}
+
 	p.startTime = time.Now()
 
 	// Before launching the writeHandler goroutine, we send any channel