Allow resolvers for specific TLDs

[pataquets]

My use case: currently using 1.1.1.1 with success. Would love to add some of the OpenNIC resolvers to also resolve their TLDs. I'm several ms away from any of them and configuring any as my main resolver would slow down every query, wheter it's a OpenNIC TLD domain or not.

[saradickinson]

This is similar to both #264 and #32 i.e. to provide config rules to forward specific requests to a specific resolvers.

Since this would require using an encrypted transport to forward, it requires dev work in the getdns library so not possible yet but am linking this issue to those previous requests as this is another use case.