Add HttpClient property to the IKubernetes interface (#548)

* Add HttpClient property to the Kubernetes interface

* PR feedback

src/KubernetesClient/IKubernetes.HttpClient.cs

@@ -0,0 +1,12 @@
+using System.Net.Http;
+
+namespace k8s
+{
+    public partial interface IKubernetes
+    {
+        /// <summary>
+        /// Gets the <see cref="HttpClient"/> used for making HTTP requests.
+        /// </summary>
+        HttpClient HttpClient { get; }
+    }
+}