Add ways to filter (or hook) request/response objects

[kanghyojun]

For more flexibility on transport layer, we need ways to filter (or hook) request/response objects:

• RPC client should take a sort of hook callbacks that filter/map request/response objects.
  • For example, RPC client could add extra headers for custom authentication process through a hook on request objects.
• RPC server should take a sort of middlewares that filter/map request/response objects.
  • For example, RPC server could add authentication process through verifying request headers on a hook.

[kanghyojun]

DONE

[dahlia]

It actually isn't done yet. We thought there should be 4 extension points i.e. (requests, response) × (client, server), but there currently are only two hooks (Client.make_request() and WsgiApp.make_response()).

In particular, response hook on client-side is necessary to extend transport layer. Because there's no extension point to urllib.request (that Client uses in the underlying) whereas server-side hooks can be replaced by WSGI middlewares. In fact, we are okay even if there are no server-side hooks.