Sending UniqueNode as top level message fails to serialize

We used to use protobuf for it, which how clashes with our DA serialization infra...
We probably should give up on mixing proto and codable for now...

Reproducer:

```swift
distributed actor X {
  distributed func test(node: UniqueNode) {} // will boom
}

try await X(actorSystem: actorSystem).test(node: actorSystem.cluster.uniqueNode)
```

