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/gcp/client.rs

@@ -3,8 +3,8 @@ pub mod api {
 }
 
 use api::{publisher_client::PublisherClient, ListTopicsRequest};
-use http::header::HeaderValue;
 use tonic::{
+    metadata::MetadataValue,
     transport::{Certificate, Channel, ClientTlsConfig},
     Request,
 };
@@ -23,7 +23,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .ok_or("Expected a project name as the first argument.".to_string())?;
 
     let bearer_token = format!("Bearer {}", token);
-    let header_value = HeaderValue::from_str(&bearer_token)?;
+    let header_value = MetadataValue::from_str(&bearer_token)?;
 
     let certs = tokio::fs::read("examples/data/gcp/roots.pem").await?;
 
@@ -32,14 +32,15 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .domain_name("pubsub.googleapis.com");
 
     let channel = Channel::from_static(ENDPOINT)
-        .intercept_headers(move |headers| {
-            headers.insert("authorization", header_value.clone());
-        })
         .tls_config(tls_config)
         .connect()
         .await?;
 
-    let mut service = PublisherClient::new(channel);
+    let mut service = PublisherClient::with_interceptor(channel, move |mut req: Request<()>| {
+        req.metadata_mut()
+            .insert("authorization", header_value.clone());
+        Ok(req)
+    });
 
     let response = service
         .list_topics(Request::new(ListTopicsRequest {