Support proxy-url for k8s client

[rmannibucau]

See https://kubernetes.io/docs/tasks/extend-kubernetes/socks5-proxy-access-api/
Right now proxy-url is not mapped nor handled within the httpclient.

[fpapon]

We need to:

• use the proxy-url if defined in the kubeconfig
• or use a new cli param --proxy-url
  to configure the HttpClient and set the Proxy type in the selector according to the proxy address protocol:
  https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/net/Proxy.java#L59