Consider supporting a list of url in attributionsrc on anchors/window.open #718

This would be useful for cases similar to #696, where only the first adtech knows the other reporting origins which may want to register for a specific click.

Moving these flows into a redirect chain involves changes from all reporting origins to perform a 302 redirect, rather than just the initial reporting origin making a change.

Note that today the ping attribute supports a space separated list of urls, which behaves similarly to the attributionsrc attribute in terms of sending background requests.