.Net: Added shared classes (#58)

* Added Throw shared class

* Added Experimental attribute

* Added IsExternalInit

* Added TrimAttributes

* Small update

* Small update

* Suppress warning

* Added comment

* Updated LegacySupport.props

* Small updates

dotnet/src/LegacySupport/TrimAttributes/RequiresUnreferencedCodeAttribute.cs

@@ -0,0 +1,39 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace System.Diagnostics.CodeAnalysis;
+
+/// <summary>
+/// Indicates that the specified method requires dynamic access to code that is not referenced
+/// statically, for example through <see cref="System.Reflection"/>.
+/// </summary>
+/// <remarks>
+/// This allows tools to understand which methods are unsafe to call when removing unreferenced
+/// code from an application.
+/// </remarks>
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Constructor | AttributeTargets.Class, Inherited = false)]
+[ExcludeFromCodeCoverage]
+internal sealed class RequiresUnreferencedCodeAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="RequiresUnreferencedCodeAttribute"/> class
+    /// with the specified message.
+    /// </summary>
+    /// <param name="message">
+    /// A message that contains information about the usage of unreferenced code.
+    /// </param>
+    public RequiresUnreferencedCodeAttribute(string message)
+    {
+        Message = message;
+    }
+
+    /// <summary>
+    /// Gets a message that contains information about the usage of unreferenced code.
+    /// </summary>
+    public string Message { get; }
+
+    /// <summary>
+    /// Gets or sets an optional URL that contains more information about the method,
+    /// why it requires unreferenced code, and what options a consumer has to deal with it.
+    /// </summary>
+    public string? Url { get; set; }
+}