feat(tonic): add Request and Response extensions (#642)

Adds `tonic::Extensions` which is a newtype around `http::Extensions`.

Request extensions can be set by interceptors with
`Request::extensions_mut` and retrieved from RPCs with
`Request::extensions`. Extensions can also be set in tower middleware
and will be carried through to the RPC.

Since response extensions cannot be set by interceptors the main use
case is to set them in RPCs and retrieve them in tower middlewares.
Figured that might be useful.

Fixes https://github.com/hyperium/tonic/issues/255

examples/src/interceptor/server.rs

@@ -16,6 +16,9 @@ impl Greeter for MyGreeter {
         &self,
         request: Request<HelloRequest>,
     ) -> Result<Response<HelloReply>, Status> {
+        let extension = request.extensions().get::<MyExtension>().unwrap();
+        println!("extension data = {}", extension.some_piece_of_data);
+
         let reply = hello_world::HelloReply {
             message: format!("Hello {}!", request.into_inner().name),
         };
@@ -40,7 +43,17 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 /// This function will get called on each inbound request, if a `Status`
 /// is returned, it will cancel the request and return that status to the
 /// client.
-fn intercept(req: Request<()>) -> Result<Request<()>, Status> {
+fn intercept(mut req: Request<()>) -> Result<Request<()>, Status> {
     println!("Intercepting request: {:?}", req);
+
+    // Set an extension that can be retrieved by `say_hello`
+    req.extensions_mut().insert(MyExtension {
+        some_piece_of_data: "foo".to_string(),
+    });
+
     Ok(req)
 }
+
+struct MyExtension {
+    some_piece_of_data: String,
+}