Add support to PPID in <amp-ad type="doubleclick"> component

## Describe the new feature or change to an existing feature you'd like to see

[Publisher provided identifier (PPID)](https://support.google.com/admanager/answer/2880055) is a widely used feature by publishers using Google Ads, to send Google Ad Manager an identifier for use in frequency capping, audience segmentation and targeting, sequential ad rotation and other audience-based ad delivery controls across devices.

Currently there's no way of passing PPID to the component.

## Describe alternatives you've considered

Discussing this with folks from the AMP team, the following solution was proposed:

"Provide support via [remote config](https://github.com/ampproject/amphtml/blob/master/extensions/amp-a4a/rtc-documentation.md), since the AMP page would need to make a callout to the publisher to learn the publishers ID for this user.
Then, the publisher could provide url and return value as response body and would need to consider if domain of call-out should be restricted to canonical page URL."

## Additional context

Forbes requested this feature as part of their mobile AMP-First site.

Start with the component and the amp-a4a remote config documentation linked in the issue. Determine how a publisher-provided identifier could be fetched and passed to Google Ad Manager, including whether the callout domain must be restricted to the canonical page URL. Done means PPID can be supplied through the proposed remote-config flow for doubleclick ads.

Written by the indexing model from the issue text.