HTTP service abstraction

[SimonSapin]

Currently we provide:

• A full server that listens for a network requests of various kinds, typically for production use
• A Tower Service for handling GraphQL requests with high-level request and response objects, typically for use in tests

It would make sense to provide the intermediate abstraction of Tower service at the HTTP level, where request and response bodies are raw bytes. This could potentially be run in a different HTTP server.

For 1.0 we want to at least have the separation internally, though perhaps not yet provide a public API for it.