add a JSON manipulation plugin stage

this stage will sit between the router stage and supergraph stage, and allow request and response manipulation in JSON format, without needing explicit buffering and deserialization at the router stage, and working with data that does not serialize cleanly to GraphQL requests and response objects.

apollo-router/src/services/mod.rs

@@ -29,6 +29,7 @@ pub(crate) mod http;
 pub(crate) mod layers;
 pub(crate) mod new_service;
 pub(crate) mod query_planner;
+pub(crate) mod json;
 pub mod router;
 pub mod subgraph;
 pub(crate) mod subgraph_service;