Allow to load Custom Fonts defined via CSS async

### Description

AMP Custom Fonts loaded via Link tags and CSS files are render blocking and it could make it harder for AMP pages to achieve a good LCP.

https://amp.dev/documentation/guides-and-tutorials/develop/style_and_layout/custom_fonts/

Example: https://webpagetest.org/video/compare.php?tests=210903_AiDcAH_5681b3d54e94822f085afe0c25282087-r:3-c:0

Is there an approved way already to load CSS Fonts files asynchronously?
This might create a FCP bottleneck towards a good LCP for some publishers.

### Alternatives Considered

Provide a way to load CSS Custom Fonts async.

### Additional Context

_No response_