add config to solve by custom dns server

[Smityz]

solve: https://github.com/tikv/client-go/issues/829

I have provided a tool for client-cpp that allows for the customization of DNS server addresses. This way, by only exposing the coreDNS address of the k8s cluster, it is possible to achieve client access to tikv across k8s clusters.

This PR depends on https://github.com/tikv/client-rust/pull/414