Add an HTTP API

[aajtodd]

Define and implement an HTTP API as an optional feature of the crate. Possibly with an associated CLI for playing with it. The API would support basic policy lifecycle operations (e.g. CRUD, attach/detach, etc) as well as authorization decisions/checks.

This API would potentially be used as part of a larger ecosystem (e.g. Auth Proxy server that sits in front of another HTTP endpoint).

This may likely belong in another crate but I want to capture all my thoughts somewhere.