GreenField: Store OnChain Payment Method (#2157)

* GreenField: Store set payment methods

* add swagger docs

* fix swagger

BTCPayServer.Client/Models/OnChainPaymentMethodPreviewResultData.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+
+namespace BTCPayServer.Client.Models
+{
+    public class OnChainPaymentMethodPreviewResultData
+    {
+        /// <summary>
+        /// a list of addresses generated by the derivation scheme
+        /// </summary>
+        public IList<OnChainPaymentMethodPreviewResultAddressItem> Addresses { get; set; } =
+            new List<OnChainPaymentMethodPreviewResultAddressItem>();
+
+        public class OnChainPaymentMethodPreviewResultAddressItem
+        {
+            /// <summary>
+            /// The key path relative to the account key path. 
+            /// </summary>
+            public string KeyPath { get; set; }
+            //The address generated at the key path
+            public string Address { get; set; }
+        }
+    }
+}