feat: Add gRPC interceptors (#232)

This change introduces proper gRPC interceptors that are avilable
regardless of the transport used. Each codegen service now produces an
additional method called `with_interceptor` that accepts a
`Interceptor`.

All examples have been updated to use this new style and interop has a
custom `tower::Service` middleware to echo the headers. There is also a
new `interceptor` example that shows basic usage.

BREAKING CHANGE: removed `interceptor_fn` and `intercep_headers_fn` from `transport` in favor of using `tonic::Interceptor`.

examples/src/interceptor/client.rs

@@ -0,0 +1,34 @@
+use hello_world::greeter_client::GreeterClient;
+use hello_world::HelloRequest;
+use tonic::{transport::Endpoint, Request, Status};
+
+pub mod hello_world {
+    tonic::include_proto!("helloworld");
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let channel = Endpoint::from_static("http://[::1]:50051")
+        .connect()
+        .await?;
+
+    let mut client = GreeterClient::with_interceptor(channel, intercept);
+
+    let request = tonic::Request::new(HelloRequest {
+        name: "Tonic".into(),
+    });
+
+    let response = client.say_hello(request).await?;
+
+    println!("RESPONSE={:?}", response);
+
+    Ok(())
+}
+
+/// This function will get called on each outbound request. Returning a
+/// `Status` here will cancel the request and have that status returned to
+/// the caller.
+fn intercept(req: Request<()>) -> Result<Request<()>, Status> {
+    println!("Intercepting request: {:?}", req);
+    Ok(req)
+}