Idea for part of #31.

Right now, we have no return value, so this can be very simple.

When the browser fetches any resource:

GET /some-resource
Host: intermediary1.example

The response can include a header field.

200 OK
Save-Impression: conversionSite=advertiser.example;histogramIndex=2;\
  filterData=12;lifetimeDays=7
Content-Type: image/jpeg

AD DATA

The browser infers the intermediary identity from the origin of the request.

If we want to define a response, then we can use a link relation trick, which I'll detail in other issues.