Following the discussion #5442, we want to introduce a new amp-ad impl to support simple image ads served from a custom server. It will not live in an 3P iframe, but rather have simple structure like <a><img></a>. It will load the image src URL and click landing page URL via a single XHR

The tentative HTML will look like:

<amp-ad type="custom" 
    width="300px" height="250px"
    data-url="https://custom.adserver.com/loadad">
</amp-ad>

Implementation plan:

• Introduce a new element impl AmpAdCustom at /extensions/amp-ad/0.1/amp-ad-custom.js.
• Add element upgrade diversion point inside upgradeCallback of /extensions/amp-ad/0.1/amp-ad.js.

@cramforce please confirm if the above plan makes sense
@xgretsch thank you for volunteering. let me know if there's anything unclear.