fix: define IsExternalInit to resolve CS0518 error in IntOrString

Author: tg123

src/KubernetesClient.Classic/IsExternalInit.cs

@@ -0,0 +1,5 @@
+// IntOrString.cs(7,36): error CS0518: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported
+namespace System.Runtime.CompilerServices
+{
+    internal static class IsExternalInit { }
+}