sketch what middleware will look like

We don't need to implement middleware for 1.0, but it would be useful to have at least a rough sketch of what it will look like so that we know that it will be possible with both the sync and async interceptor interfaces.

For an example of a sketch of this for node.js see this branch:

https://github.com/cprice404/client-sdk-javascript/commit/d1c68c3a934fffc2953fa48d757f3c6c9e71834c