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`.

interop/src/lib.rs

@@ -9,13 +9,7 @@ pub mod pb {
     include!(concat!(env!("OUT_DIR"), "/grpc.testing.rs"));
 }
 
-use http::header::{HeaderMap, HeaderName, HeaderValue};
-use http_body::Body;
-use std::{
-    default, fmt, iter,
-    pin::Pin,
-    task::{Context, Poll},
-};
+use std::{default, fmt, iter};
 
 pub fn trace_init() {
     let sub = tracing_subscriber::FmtSubscriber::builder()
@@ -147,41 +141,3 @@ macro_rules! test_assert {
         }
     };
 }
-
-pub struct MergeTrailers<B> {
-    inner: B,
-    trailer: Option<(HeaderName, HeaderValue)>,
-}
-
-impl<B> MergeTrailers<B> {
-    pub fn new(inner: B, trailer: Option<(HeaderName, HeaderValue)>) -> Self {
-        Self { inner, trailer }
-    }
-}
-
-impl<B: Body + Unpin> Body for MergeTrailers<B> {
-    type Data = B::Data;
-    type Error = B::Error;
-
-    fn poll_data(
-        mut self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-    ) -> Poll<Option<Result<Self::Data, Self::Error>>> {
-        Pin::new(&mut self.inner).poll_data(cx)
-    }
-
-    fn poll_trailers(
-        mut self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-    ) -> Poll<Result<Option<HeaderMap>, Self::Error>> {
-        Pin::new(&mut self.inner).poll_trailers(cx).map_ok(|h| {
-            h.map(|mut headers| {
-                if let Some((key, value)) = &self.trailer {
-                    headers.insert(key.clone(), value.clone());
-                }
-
-                headers
-            })
-        })
-    }
-}