Add ProcessedAt property to Transaction DTO

[MatejMarecek]

The DTO is missing processed_at property.

The property is not documented but it in the provided example JSON.

The code to add to Transaction.cs

/// <summary>
/// This property is undocumented by Shopify.
/// </summary>
[JsonProperty("processed_at")]
public DateTimeOffset? ProcessedAt { get; set; }