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.
do you need our service explicitly? i think it still makes sense to have
this to get more explicit errors and understand what is going on. having
a dead service you should not try to use sounds flaky