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/uds/client.rs

@@ -5,11 +5,10 @@ pub mod hello_world {
 }
 
 use hello_world::{greeter_client::GreeterClient, HelloRequest};
-use http::Uri;
 use std::convert::TryFrom;
 #[cfg(unix)]
 use tokio::net::UnixStream;
-use tonic::transport::Endpoint;
+use tonic::transport::{Endpoint, Uri};
 use tower::service_fn;
 
 #[cfg(unix)]