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`.

tonic-build/src/client.rs

@@ -34,6 +34,11 @@ pub(crate) fn generate(service: &Service, proto: &str) -> TokenStream {
                     Self { inner }
                 }
 
+                pub fn with_interceptor(inner: T, interceptor: impl Into<tonic::Interceptor>) -> Self {
+                    let inner = tonic::client::Grpc::with_interceptor(inner, interceptor);
+                    Self { inner }
+                }
+
                 #methods
             }