Enable Google Ad Manager Publisher first-party IDs

### Summary

Publisher first-party IDs are a set of first-party cookies set within the publisher’s own site that enhance ad serving by enabling frequency-capping in reservation campaigns, buyer frequency capping, as well as ads personalization on programmatic traffic.

To enable first-party IDs and see more details about them, visit [Help Center: Enable first-party user identifiers](https://support.google.com/admanager/answer/10650804).

The proposal here would facilitate the setting of publisher first-party IDs on the publisher’s AMP page for opted-in publishers.

### Design Document

There are two main components to this proposal: cooking reading and cookie writing.

Instructions to write the new cookies will be delivered via ad response to amp-ad-network-doubleclick-impl elements via HTTP header ‘amp-ff-set-cookies’. Additionally, user opt-out preferences will also be delivered via ad response, and if so opted-out, all Google Ad Manager publisher first-party IDs will be deleted (specifically, the ‘__gads’ and ‘__gpi’ cookies).

Cookie reading will occur at ad request time when request parameters are generated. Reading will be conditioned on having the appropriate user consent.

### Motivation

Supporting publisher first-party IDs will help move ad delivery on AMP pages closer to feature parity with non-AMP pages, and thereby help publishers better monetize their AMP pages.

### Alternative Solutions

N/A

### Launch Tracker

N/A

### Notifications

/cc @ampproject/wg-approvers