feat(gam): support fluid ad unit sizing

[miguelpeixe]

Implements support for GAM's fluid ad unit size. Fluid is supported multisized or not for both AMP and AMP Plus.

00cee00 added a fix to fluid rendering on AMP Plus. GPT currently sets the iframe with min-width set to 100% and width attribute set to 0. This causes the iframe to be rendered with 0 width. The fix listens to rendered slot and updates the width if native content is identified.

How to test changes

1. Check out this branch, feat(ads): fluid ad unit sizing newspack-plugin#1181, and create 4 new ad units:
   1. In legacy mode and with uploaded credentials:
      1. With a regular fixed size and fluid option checked
      2. Without any size and fluid option checked
2. Make sure you have an active GAM campaign to display fluid and the selected fixed size
3. Add the ad units to a page
4. With AMP Plus disabled:
   1. Make sure the multisized ad units are properly displayed and rotate between the fixed and fluid content
   2. Make sure the fluid-only ad unit is properly displayed
5. With AMP Plus enabled, refresh the page and make sure it behaves the same

Note: There's a known issue with AMP that sometimes won't render ads that are loaded on the viewport. Make sure to put the ads outside of the initial viewport and will require you to scroll to the ad for it to properly load.

Closes #195

[miguelpeixe]

Thank you, @adekbadek ! Do you mind taking another quick look? I had to merge #202

[matticbot]

🎉 This PR is included in version 1.18.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.18.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀