Adding design docs for declarative proxy handlers

What you are describing here is closer to a compiler / code generator. I would suggest to keep things simpler for now by assembling pre-defined building blocks:

- send_request handler that implements context-sensitive substitution on the request structure & calls restbase.request
- response handler that checks conditions & hooks up the responses; return can probably be implemented inline

I *think* this can be implemented by hooking up promises with appropriately bound closures. (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind).

Generally, we'd like to only allow top-level returns in the first request tree. All other parallel operations are executed without any returns.