Implement RPCs with responses

[philpax]

#240 allows for one-way messages, but we should support RPCs with responses:

[messages.hello]
name = "Hello"
description = "Sent when a client joins the server, then sent back from the server"
fields = { text = "String", source_reliable = "Bool" }
return = "String"

Implementor's side:

#[rpc(messages::Hello)]
fn hello(text: String, source_reliable: bool) -> String {
  format!("{text} {source_reliable}")
}

Guest side:

async fn main() {
  let response = messages::Hello::call(Target::NetworkReliable, "Hello world!", true).await;
  println!("{response}");
}