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/Cargo.toml

@@ -86,27 +86,30 @@ path = "src/uds/client.rs"
 name = "uds-server"
 path = "src/uds/server.rs"
 
+[[bin]]
+name = "interceptor-client"
+path = "src/interceptor/client.rs"
+
+[[bin]]
+name = "interceptor-server"
+path = "src/interceptor/server.rs"
+
 [dependencies]
 tonic = { path = "../tonic", features = ["tls"] }
 prost = "0.6"
-
 tokio = { version = "0.2", features = ["rt-threaded", "time", "stream", "fs", "macros", "uds"] }
-futures = { version = "0.3", default-features = false, features = ["alloc"]}
+futures = { version = "0.3", default-features = false, features = ["alloc"] }
 async-stream = "0.2"
-http = "0.2"
-tower = "0.3" 
-
+tower = "0.3"
 # Required for routeguide
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 rand = "0.7"
-
 # Tracing
 tracing = "0.1"
-tracing-subscriber =  { version = "0.2.0-alpha", features = ["tracing-log"] }
+tracing-subscriber = { version = "0.2.0-alpha", features = ["tracing-log"] }
 tracing-attributes = "0.1"
 tracing-futures = "0.2"
-
 # Required for wellknown types
 prost-types = "0.6"