Prestitial ad support for amp-ad component

### Description

Hello,

I see that the Prestitial ad or full page ad format is not implemented yet in AMP. It is very common in mobile world and would be nice to see it created using standards and features of AMP.

The features:

When the page loads, the first thing displayed should be the full page overlay with ad, the page flash to screen before the ad should not happen(this is required by Google to show the ad first). If the ad not loaded due to some server preferences, for example the geolocation restrictions, the ad will not be shown, or it will be hidden just after noticing that the ad is empty.

If the ad is loaded the user should be able to dismiss it, or it can be hidden after some time.

Please let me know if this type of ad is going agains standards of AMP, is it acceptable, and if it is not going to be implemented in some near feature, how this type of functionality do you suggest for AMP pages to be created.

Any guide for developer would be much appreciated.

### Alternatives Considered

We have this type of ad working on regular html. I see that it is possible to display Iframe in AMP with regular jquery and ads javascript logic all as it is now in html. But loading this content in AMP on each page load is terrible solution, and it will make the page too slow thus defeating the general meaning of AMP , the speed.

So for now I did not implement any solution yet, and I am looking for native amp component with these features, and in general to get any advice about the possibilities.

### Additional Context

_No response_

The issue does not name an AMP component file, test, or entry point. Start by reviewing AMP's existing ad components and relevant standards to determine whether prestitial behavior is permitted and where such a component would fit. Done would require an agreed scope covering loading, fallback, dismissal, timing, and performance behavior.

Written by the indexing model from the issue text.