add experimental features join-ad-interest-group and run-ad-auction

[qingxinwu]

Features join-ad-interest-group and run-ad-auction are enabled by default for all frames. The default behavior will be changed to same-origin iframes only in near future, before third party cookie is deprecated.
There is a flag AdInterestGroupAPIRestrictedPolicyByDefault (--enable-features=AdInterestGroupAPIRestrictedPolicyByDefault) which will disable the two features for cross-origin iframes by default. Then cross-origin iframes need to specify an allow attribute on the iframe, e.g. <iframe src="some_page.html" allow="join-ad-interest-group"></iframe>, in order to access the features.

[mikewest]

Would you be willing to put up a PR against https://github.com/w3c/webappsec-permissions-policy/blob/main/features.md listing these features and linking to documentation explaining their function?

(Edit: I am dumb. That's what this is. :) )

The explainer linked in this PR doesn't list either of these feature names. It would be ideal to update that document to define the feature, and its default allowlist (for example: https://wicg.github.io/idle-detection/#api-permissions-policy).

[mikewest]

(Also, for IPR reasons, you'll need to link your GitHub account with a W3C account, and follow the (Google internal) instructions at go/github to get into the right GitHub group.)

[qingxinwu]

The explainer is updated to mention the feature names. It also talks about the ideal default allowlist, but we think the allowlist details fit more in a spec (which is still in progress) than in the explainer. Please let me know if there's any question. Thanks!

[clelland]

Hey @qingxinwu - would it makes sense at this point to link these features to the spec instead, now that the feature definition has been integrated there?

[clelland]

Ping @qingxinwu :)

[qingxinwu]

Hey @qingxinwu - would it makes sense at this point to link these features to the spec instead, now that the feature definition has been integrated there?

Updated to link to the spec.
Not sure why I missed this comment, sorry about that.

[clelland]

Thanks!