Allow dynamic api calls for static pages

[tumenbaev]

Right now next pushes everyone to use static rendering however there're some use cases that block us from doing it. So my proposal is to give an api that allows to call dynamic api like cookies, headers and maybe others without opting out of static rendering.

Here are some examples:

• passing custom tracing id from headers to fetch request made from server for better observability (this would also break all the caches, but it's another story). Right now, the only way is to call headers which opts out of static render or stop using such ids which is inconvenient.
• using cookies to authorise fetch requests from server. Passing cookies doesn't mean that the response is unique for used and it could be used for access control instead.

The way to solve it could be some api that allows passing data from middleware to the page. Maybe it could be added to NextResponse like NextResponse.redirect(url, { additionalData }).
Or it could be some new way to call dynamic api like unsafeCookies() to get cookies that would be shared across requests from different users.