Support for gRPC-JSON Transcoding

Many gRPC services need to be exposed as RESTful JSON APIs to support legacy clients, frontend applications, or public APIs that do not support gRPC natively. Currently, users often have to set up a separate proxy (like Envoy) or use a separate gateway service just to handle this translation.

I would like to propose adding support for gRPC-JSON Transcoding directly within spring-grpc. This feature allows mapping JSON/HTTP requests to gRPC methods using configuration (often via google.api.http annotations in Protobuf).

This concept is well-documented in Google Cloud Endpoints: Transcoding HTTP/JSON to gRPC.

Currently, other ecosystems and frameworks within the Java landscape provide this functionality:

• Armeria: Provides built-in support for serving gRPC and REST/JSON from the same port.
• Spring Cloud Gateway: Offers a JsonToGrpc filter factory to act as a transcoding gateway.

Having native support or a dedicated starter in spring-grpc (e.g., essentially embedded transcoding capability) would greatly simplify the architecture for Spring Boot developers who want to expose their gRPC services over HTTP/JSON without maintaining a separate Gateway component.

Start with the linked Google Cloud transcoding documentation and compare the Armeria and Spring Cloud Gateway approaches mentioned in the issue. Determine the intended native support or dedicated starter scope, including how google.api.http annotations would be handled; done requires an agreed design and implementation boundary.