Support for dead resolver to run outside of a Kubernetes cluster

Hi @sercand , 
This is a great little project, I was thinking of writing something similar from scratch before I found this project. 

Would it be possible to add a [deadResolver](https://github.com/grpc/grpc-go/blob/master/internal/resolver/dns/dns_resolver.go#L126-L130) to the kubernetes resolver ? 

this will help folks who want to run their gRPC client outside Kubernetes for testing