Moving comments over from w3c/reporting#169

In general, this API should be opt-in.

1. This introduces privacy harm, in that it opens up new channels for communication between the site and potentially 3rd parties, over types of information not currently easily captured.
2. This should be opt in, since the primary beneficiary is the web site, and not the web user.
3. These reports are particularly user-hostile, since the whole point of many interventions is that the user is trying to hide information from the site they're visiting (and the included 3 parties)
4. What information would be added to the WebExtension layer to allow extensions or similar to block Depreciation reports (independent of other POSTs / reports)?