Add configuration for logout handler

Currently, setting `user_context: enabled` will throw an exception if no proxy client is configured because of the logout handler, which requires the client. This PR solves that by adding a `user_context.logout_handler.enabled` config option, which defaults to auto, so won't be available without a proxy client.
not sure i understand the question. for me it would make sense to say i don't want to actively invalidate the cache. either because i was paranoid and don't cache the hash requests anyways, or because i don't care if a login stays kind of valid for a longer time.

though, thinking about this: if i have a cached hash, but the cookie is not valid in the backend anymore, what would happen? we get back a forbidden/request to log in - which absolutely must NOT just vary on the user hash... what is the default behaviour by symfony when a session cookie was not valid?