With support of http, https, and socks5 proxy support. We already
support configuring this via environmnet variables, but this approach
becomes inconvenient dealing with multiple clusters on different
networks, that require different proxies to connect to. Most solutions
require wrapping clients (like kubectl) in bash scripts.
Part of: https://github.com/kubernetes/client-go/issues/351
Kubernetes-commit: f3f666d5f1f6f74a8c948a5c64af993696178244