feat(build): Better support for custom codecs (#999)

BREAKING CHANGE: `CODEC_PATH` moved from const to fn

examples/build.rs

@@ -1,5 +1,4 @@
-use std::env;
-use std::path::PathBuf;
+use std::{env, path::PathBuf};
 
 fn main() {
     tonic_build::configure()
@@ -30,4 +29,30 @@ fn main() {
             &["proto/googleapis"],
         )
         .unwrap();
+
+    build_json_codec_service();
+}
+
+// Manually define the json.helloworld.Greeter service which used a custom JsonCodec to use json
+// serialization instead of protobuf for sending messages on the wire.
+// This will result in generated client and server code which relies on its request, response and
+// codec types being defined in a module `crate::common`.
+//
+// See the client/server examples defined in `src/json-codec` for more information.
+fn build_json_codec_service() {
+    let greeter_service = tonic_build::manual::Service::builder()
+        .name("Greeter")
+        .package("json.helloworld")
+        .method(
+            tonic_build::manual::Method::builder()
+                .name("say_hello")
+                .route_name("SayHello")
+                .input_type("crate::common::HelloRequest")
+                .output_type("crate::common::HelloResponse")
+                .codec_path("crate::common::JsonCodec")
+                .build(),
+        )
+        .build();
+
+    tonic_build::manual::Builder::new().compile(&[greeter_service]);
 }