Extend Permissions-Policy standard to support needs of Topics API

[dmarti]

In #60 it was decided that the appropriate way to implement Topics API opt-outs at the JS or HTML level (to support A/B testing and/or sites hosted on services that don't allow setting HTTP response headers) is to extend the Permissions-Policy spec.

This issue is for tracking the progress of Topics API required functionality in Permissions-Policy, so that Topics API users can keep up with it. @jkarlin @michaelkleber Can you comment on your progress with Permissions-Policy, and if possible include links to issues and discussion? Thank you -- I know that there are a lot of GitHub repositories to keep up with in order to follow your work, so it would be very helpful for Topics API users and testers to understand the status of this dependency.

[jkarlin]

I've had multiple discussions with permissions policy folks over the years about this issue (both for topics and other policies that it would be nice to support js opt outs for) but have not had luck. This is why I encourage developers to add support to the issue as they may be more persuasive in describing their use cases.

[chrisn]

Relevant issues: w3c/webappsec-permissions-policy#55, w3c/webappsec-permissions-policy#15