Add attribute to serialize strings as base64 encoded

Octokit/Helpers/SerializeAsBase64Attribute.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Octokit.Helpers
+{
+    /// <summary>
+    /// Attribute used to denote that a string property should be serialized as a base64 encoded string.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
+    public sealed class SerializeAsBase64Attribute : Attribute
+    {
+    }
+}