Add support for new properties on upcoming invoice retrieval

src/Stripe.net/Services/Invoices/InvoiceItemPeriodOptions.cs

@@ -0,0 +1,20 @@
+namespace Stripe
+{
+    using System.Collections.Generic;
+    using Newtonsoft.Json;
+
+    public class InvoiceItemPeriodOptions : INestedOptions
+    {
+        /// <summary>
+        /// The end of the period, which must be greater than or equal to the start.
+        /// </summary>
+        [JsonProperty("end")]
+        public DateTime? End { get; set; }
+
+        /// <summary>
+        /// he start of the period.
+        /// </summary>
+        [JsonProperty("start")]
+        public DateTime? Start { get; set; }
+    }
+}

src/Stripe.net/Services/Invoices/InvoiceUpcomingInvoiceItemOption.cs

@@ -22,5 +22,14 @@ public class InvoiceUpcomingInvoiceItemOption : INestedOptions
 
         [JsonProperty("metadata")]
         public Dictionary<string, string> Metadata { get; set; }
+
+        [JsonProperty("period")]
+        public InvoiceItemPeriodOptions Period { get; set; }
+
+        [JsonProperty("quantity")]
+        public long? Quantity { get; set; }
+
+        [JsonProperty("unit_amount")]
+        public long? UnitAmount { get; set; }
     }
 }