Permissions Policy `deferred-fetch` and `deferred-fetch-minimal`

[mingyc]

Update: The latest proposal (and implementation in Chromium) includes two new permissions policy deferred-fetch and deferred-fetch-minimal. But at the time of initial post it was only deferred-fetch.

The example usage and full explaination can be found at https://github.com/WICG/pending-beacon/blob/main/docs/fetch-later-api.md#quota-and-permissions-policy

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For the new deferred fetching (fetchLater) API (explainer, Spec PR), we would like to propose a new permissions policy deferred-fetch to help developers manage the quota for all fetchLater requests that can be fired within their documents.

The discussion happened in WICG/pending-beacon#87, and in the comment we proposed to add deferred-fetch along with its usage.

Specifically, the main motivation is that, without any constraints, every frame can queue an unlimited number of fetchLater requests, which may use up network resources. However, unlike other types of JS-initiated requests, there is no way for users to be able to terminate pending fetchLater requests, as closing a tab will simply trigger sending all fetchLater requests on it.

While setting a quota for individual sites, we would also like to prevent (1) leaking cross-site activity and (2) abusing by creating additional frames.

@noamr @annevk @yoavweiss @nicjansma