* Auth Proxy: Include additional headers as part of the cache key

Auth proxy has support to send additional user attributes as part of the
authentication flow. These attributes (e.g. Groups) need to be monitored
as part of the process in case of change.

This commit changes the way we compute the cache key to include all of the
attributes sent as part of the authentication request. That way, if we
change any user attributes we'll upsert the user information.