✨ Add amp-ad implementation for gsrv

[goadserver]

Adds gsrv as a third-party ad vendor: ad zones served by ad networks that run on GoAdserver.

<amp-ad width="300" height="250" type="gsrv"
  data-host="g2.gsrv.dev" data-ctrl="19460" data-zone="12512838">
</amp-ad>

How it works

• ads/vendors/gsrv.js validates the attributes and loads the network's ad controller from https://<data-host>/ctrl/<data-ctrl>.js into the 3p frame. data-host is the network's own serving domain, validated as a plain host name (each network on the platform serves from its own domain, the same model as the aso integration).
• The controller renders the zone into a div inside #c and reports to AMP itself:
  • renderStart({width, height}) once the ad has rendered (images loaded), with the creative's height; the width stays the slot's.
  • noContentAvailable() when the zone has no ad, or the controller cannot be loaded.
  • reportRenderedEntityIdentifier() with the zone id (no user data).
• Clicks open in a new window through the ad server's click URL.
• Publisher documentation: ads/vendors/gsrv.md.

Changes included

• 3p/vendors/gsrv.js: vendor bootstrap.
• ads/vendors/gsrv.js: main logic.
• ads/vendors/gsrv.md: publisher documentation.
• ads/_config.js: renderStartImplemented: true. There is no prefetch/preconnect entry because the host differs per network.
• extensions/amp-ad/amp-ad.md: link to the documentation.
• examples/amp-ad/ads.amp.html + ads.amp.esm.html: an example on a demo zone that always fills.
• test/unit/ads/test-gsrv.js: unit tests.

Testing

• amp lint, amp prettify, amp check-types all pass.
• amp unit --files=test/unit/{test-ads-config.js,test-integration.js},test/unit/ads/test-gsrv.js passes.
• Manual test on the local dev server at desktop and mobile widths, against the live ad servers:
  • banner and native zones render and resize;
  • the impression is logged and clicks go through the click URL;
  • a zone with no ad collapses.

Live demo of ads on AMP pages: https://www.goadserver.com/demo/amp/

Contact: support@goadserver.com

cc @ampproject/wg-monetization

[CLAassistant]

All committers have signed the CLA.