For images, don't raise a validation error for loading="lazy" or decoding="async"

[kienstra]

Summary

For images, this prevents validation errors if loading="lazy" or decoding="async", but raises an error for any other value. Uses Weston's snippet in #3938 (comment).

Fixes #3938

Using Weston's testing snippet in a Custom HTML block:

<img decoding="async" loading="eager" src="https://blog.amp.dev/wp-content/uploads/2019/12/AdobeStock_163856462-e1574813805126-1024x448.jpeg" alt="" class="wp-image-3003" width="1024" height="448">

...there is now only a validation error for loading="eager", not decoding="async":

And there's no validation error for <img decoding="async" loading="lazy" ...>, though those aren't copied into the <amp-img>:

Sanitization behavior

<img> attribute	value	Validation error?	Copied into <amp-img>?
loading	lazy	no	no
loading	anything other than lazy, like eager	yes	no
decoding	async	no	no
decoding	anything other than async, like sync	yes	no

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[kienstra]

Request For Review

Hi @westonruter,
Hope you're doing great.

Could you please review this PR when you have a chance?

Thanks, Weston!

[westonruter]

Good work 👍