Add "_when_accepted" block-data-on-consent value for <amp-ad>

## Describe the new feature or change to an existing feature you'd like to see

Hello everyone! As part of a complex mechanism that needs two different adNetworks to deliver an ``, I was looking for a way to:
* block networkA / allow networkB to send an ad request when the GDPR consent is negative/missing
* allow networkA / block networkB to send an ad request when the GDPR consent is positive
To do so, the native support of something like `` would be awesome.

Is it something that can be considered?
Thanks!

## Describe alternatives you've considered

So far, I tried to register the `` with some custom .js code but it is naturally blocked by AMP. So I was thinking about the native support of data-block-on-consent

## Additional context

The final setup would be
````

````

Start by tracing how handles the existing data-block-on-consent value and how consent states are represented. Check the related implementation and tests for consent blocking, then confirm that the proposed _when_accepted behavior distinguishes positive from negative or missing consent and that both ad configurations work as described.

Written by the indexing model from the issue text.