Add CookieFunc to update cookie based on request before writing it to http-response

[guybrush]

This way it would be possible to set cookie-attributes based on the request

[alexedwards]

@guybrush Thanks for the PR. Can you describe a real-world scenario where this is required? As far as I know, apart from Partitioned, all the cookie settings can be configured via SessionManager.Cookie already, so this would only be useful if you want to change the cookie settings on a per-request basis.