Adding support for user context for symfony HttpCache

Moving most of https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/issues/177 to the library.

*TODO*
* [ ] Documentation (see also https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/issues/178 but note that things changed since then)
same question for https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/pull/179/files#r21372777