Added license header to CertificateAuthenticationProvider cs file.

ClientRuntimes/CSharp/Microsoft.Rest.ClientRuntime.Azure.Authentication/CertificateAuthenticationProvider.cs

@@ -1,4 +1,7 @@
-using System;
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License. See License.txt in the project root for license information.
+
+using System;
 using System.Threading.Tasks;
 using Microsoft.IdentityModel.Clients.ActiveDirectory;