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.

itest/list_on_test.go

@@ -531,6 +531,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "custom message",
 		TestFunc: testCustomMessage,
 	},
+	{
+		Name:     "onion message",
+		TestFunc: testOnionMessage,
+	},
 	{
 		Name:     "sign verify message with addr",
 		TestFunc: testSignVerifyMessageWithAddr,