Support FedCM-based grants

[johannhof]

In the FedID CG we have been discussing the merits of autogranting Storage Access calls based on existing FedCM grants. Based on the positive reception of this idea we wrote up an explainer of how we think this should work from a technical perspective: https://github.com/explainers-by-googlers/storage-access-for-fedcm

Relevant for this specification is that instead of simply creating a new storage-access permission on a successful FedCM prompt, we'd update Storage Access to look at existing FedCM accounts connections to establish whether storage access can be granted without an additional prompt. Benefits to this include the ability to scope the grant to the privacy boundaries of FedCM, and avoiding two simultaneous permission grants for the user (agent) to manage.

This issue is tracking discussion and integration on the Privacy CG side.

cc @bvandersloot-mozilla @annevk

[martinthomson]

This was discussed at the last meeting and the conclusion was that there was interest, but we'd need to establish that enough implementations were prepared to deploy this. I think that we have those indications, but we will confirm.