Notion of a "session" across same-origin navigations

Currently when navigating inside the same origin, the client is removed and a new client ID is created.
With cookies being a blocked header, this makes it so that SWs don't have the knowledge to maintain information across navigations in the same session (the same session as in sessionStorage).

Maybe something in the navigation API is supposed to fix this or there's a separate issue that I didn't recognize. The issues mentioning access to the cookie storage are somewhat orthogonal to this.

The use case is maintaining some sort of MPA session state in the service worker as a cache to the cookie-based session state on a server.

A possible small-ish API that would enable this: adding a previousClientId to navigation request fetch events.

Start by examining the navigation API and service-worker navigation request fetch events described in the issue, comparing their behavior with sessionStorage and cookie storage. Determine whether a previousClientId or another mechanism should preserve state across same-origin navigations; done means documenting a decided API direction and its session semantics.