Add callback support to kryptoEcMiddleware

[directionless]

Problem

Sometimes we see a browser able to make requests to http://localhost, but unable to meaningfully work with the response. This breaks agent detection in the device trust flow.

Solution

One solution is to setup a simple callback. The intent here, is to allow K2 to set a URL to post back to, and then kryptoEcMiddleware can post the response to it.

This should be reasonable secure -- the callback URL is inside the krypto box, and the response is the same as what we'd send via PNG. eg: it's all opaque crypto data. There are a couple of small exceptions...

1. We include the user agent headers
2. we send errors

I first tried implementing this in middleware, but it was messy. First, because the url comes out of the box, so that has to be passed from the inner layer up to the outer layer. Second, because the outer layer works with readers, so there's a bunch of extra io teeing. It's much simpler this way.

[directionless]

@James-Pickett This is in draft since the corresponding K2 side isn't merged, but how's the shape look to you?

[James-Pickett]

This code looks good to me. Would like a little clarification on the goal of this thought, is it just so we can report errors back up to k2?