Add Greenfield invoice refund endpoint (#4238)

* Add Greenfield invoice refund endpoint

See discussion here: https://github.com/btcpayserver/btcpayserver/discussions/4181

* add test

* add docs

BTCPayServer.Client/Models/RefundInvoiceRequest.cs

@@ -0,0 +1,25 @@
+#nullable enable
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+
+namespace BTCPayServer.Client.Models
+{
+    public enum RefundVariant
+    {
+        RateThen,
+        CurrentRate,
+        Fiat,
+        Custom,
+        NotSet
+    }
+
+    public class RefundInvoiceRequest
+    {
+        public string? Name { get; set; } = null;
+        public string? Description { get; set; } = null;
+        [JsonConverter(typeof(StringEnumConverter))]
+        public RefundVariant RefundVariant { get; set; } = RefundVariant.NotSet;
+        public decimal CustomAmount { get; set; } = 0;
+        public string? CustomCurrency { get; set; } = null;
+    }
+}