feat(transport): Add service multiplexing/routing (#99)

* feat(transport): Add service multiplexing/routing

This change introduces a new "router" built on top of
`transport::Server` that allows one to run multiple
gRPC services on the same socket.

```rust
Server::builder()
    .add_service(greeter)
    .add_service(echo)
    .serve(addr)
    .await?;
```

There is also a new `multiplex` example showcasing
server side service multiplexing and client side
service multiplexing.

BREAKING CHANGES: `Server::serve` is now crate private
and all services must be added via `Server::add_service`.
Codegen also returns just a `Service` now instead of a
`MakeService` pair.

Closes #29

Signed-off-by: Lucio Franco [email protected]

tonic-examples/src/multiplex/client.rs

@@ -0,0 +1,37 @@
+pub mod hello_world {
+    tonic::include_proto!("helloworld");
+}
+
+pub mod echo {
+    tonic::include_proto!("grpc.examples.echo");
+}
+
+use echo::{client::EchoClient, EchoRequest};
+use hello_world::{client::GreeterClient, HelloRequest};
+use tonic::transport::Endpoint;
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let channel = Endpoint::from_static("http://[::1]:50051").channel();
+
+    let mut greeter_client = GreeterClient::new(channel.clone());
+    let mut echo_client = EchoClient::new(channel);
+
+    let request = tonic::Request::new(HelloRequest {
+        name: "Tonic".into(),
+    });
+
+    let response = greeter_client.say_hello(request).await?;
+
+    println!("GREETER RESPONSE={:?}", response);
+
+    let request = tonic::Request::new(EchoRequest {
+        message: "hello".into(),
+    });
+
+    let response = echo_client.unary_echo(request).await?;
+
+    println!("ECHO RESPONSE={:?}", response);
+
+    Ok(())
+}