Add IsExternalInit type, BaseCmdlet class, and SessionStatus enum

Introduce the IsExternalInit type for C# 9 records compatibility in .NET Standard 2.1. Implement BaseCmdlet class to provide common functionality for cmdlets, including methods for writing results and handling async operations. Add SessionStatus enum to represent API session states.

classlib/IsExternalInit.cs

@@ -0,0 +1,23 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// This file provides the IsExternalInit type required for using C# 9 records
+// in projects targeting .NET Standard 2.1 or earlier.
+
+#if !NET5_0_OR_GREATER
+
+namespace System.Runtime.CompilerServices
+{
+    using System.ComponentModel;
+
+    /// <summary>
+    /// Reserved to be used by the compiler for tracking metadata.
+    /// This class should not be used by developers in source code.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    internal static class IsExternalInit
+    {
+    }
+}
+
+#endif