Add license info for Azure plugin. (Azure#45863)

Author: live1206

eng/packages/http-client-csharp/generator/Azure.Generator/src/AzureClientPlugin.cs

@@ -27,4 +27,12 @@ public AzureClientPlugin(GeneratorContext context) : base(context) { }
     public override void Configure()
     {
     }
+
+    /// <summary>
+    /// Customize the license string for Azure client SDK.
+    /// </summary>
+    public override string LicenseString => """
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+""";
 }