Add support for enabling jetstream globally

[wallyqs]

Currently it is needed to enable jetstream at the top level as part of the main config that is not managed by the nats-rest-config-proxy which makes it unable to be able to validate the jetstream config before runtime.

jetstream {
  max_file = 20GB
  max_mem = 2GB
  store_dir = '/var/lib/jetstream/data'
}

include 'auth.conf'