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.

log.go

@@ -46,6 +46,7 @@ import (
 	"github.com/lightningnetwork/lnd/monitoring"
 	"github.com/lightningnetwork/lnd/msgmux"
 	"github.com/lightningnetwork/lnd/netann"
+	"github.com/lightningnetwork/lnd/onionmessage"
 	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
 	"github.com/lightningnetwork/lnd/peer"
 	"github.com/lightningnetwork/lnd/peernotifier"
@@ -212,6 +213,7 @@ func SetupLoggers(root *build.SubLoggerManager, interceptor signal.Interceptor)
 		root, paymentsdb.Subsystem, interceptor, paymentsdb.UseLogger,
 	)
 
+	AddSubLogger(root, onionmessage.Subsystem, interceptor, onionmessage.UseLogger)
 }
 
 // AddSubLogger is a helper method to conveniently create and register the