DelegatingHandler now works with Kubernetes clients

[tg123]

this enables user side create a Kubernetes client with all kinds of generated ctors:

• public Kubernetes(ServiceClientCredentials credentials, params DelegatingHandler[] handlers)
• public Kubernetes(ServiceClientCredentials credentials, HttpClientHandler rootHandler, params DelegatingHandler[] handlers)
• ...

watcher handler will be applied to them automatically. thus, user side will no long binded to KubernetesClientConfiguration ctor.

Also KubernetesClientConfiguration ctor is now support DelegatingHandler[]

var handler1 = new DummyHandler();
var handler2 = new DummyHandler();

var client = new Kubernetes(KubernetesClientConfiguration.BuildConfigFromConfigFile(), handler1, handler2);

This PR also remove a confusing ctor KubernetesClientConfiguration which was replace by factory style ctor BuildConfigFromConfigFile(string masterUrl = null, string kubeconfigPath = null)

[brendandburns]

LGTM, but needs rebase.