[scanner integration] Flag third party cookies in scan results

[eloquence]

As part of epic #488, we should flag any third party cookies set as part of serving a landing page. First party cookies are not necessarily problematic, but third party cookies are indicative of embedded resources that don't belong in a landing page.

This is a must-have feature for the initial integration, because the current cookie detection is inappropriately broad (e.g., NYT would fail, even though it only sets two harmless cookies), and we do want to be able to detect serious cookie-related issues.

[eloquence]

This task isn't very well thought out, closing for now.

Third party cookies would be set as part of third party assets embedded in the page. Such third party assets should already trigger a severe warning (#506) so it's not clear that we gain much from this additional check, nor is it clear that it could be easily done before work on #506.