Adding support for user context for symfony HttpCache

Moving most of https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/issues/177 to the library.

* [x] Documentation (see also https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/issues/178 but note that things changed since then)
hm. "Symfony Reverse Proxy from the http-kernel component"? i want to point out that its about a component ,and not the symfony framework.