How to customize the upstream resolver?

Users of solutions like Pi-hole® are used to be able to choose an upstream resolver. While NextDNS can be thought as a Pi-hole® in the cloud, it does not work the same way. NextDNS is itself a DNS recursive resolver, so it does not make sense to setup a different upstream. Keep in mind that if we would allow custom upstream resolver, you wouldn’t benefit from our shared cache and all your DNS request would have to pay for double latency (you → NextDNS → upstream resolver).