Support measurement of non-paid/organic clicks

[johnwilander]

An advertiser reached out to me and said they'd love to be able to also measure non-paid incoming clicks, sometimes referred to as organic clicks. This would mean the browser accepts all incoming navigations resulting from cross-site navigations and stores them with a blank source ID. The resulting attribution report would somehow signal that it is such an organic attribution, for instance by omitting the source_id field and potentially changing the source_engagement_type.