(PE-5594) Port ring-proxy

This PR ports the ring-proxy code into the ring-middleware codebase and modifies it to use clj-http-client.
Nitpick, but, if you're going to have a `common` namespace, maybe the two utility functions above should go there?  I think it'd be a good goal for this core namespace to only contain things that we consider "API"; that we expect users to call from outside of this code base.  I *think* the only thing that qualifies so far would be the `wrap-proxy` function below.