Greenfield: Refactor app endpoints (#6051)

* Greenfield: Refactor app endpoints

- Do not change unset data
- Clean up difference between request (template) and data (items/perks)
- Add missing properties (form id and custom tip percentage)
- Update docs

* Revert ToSettings changes in GreenfieldAppsController

BTCPayServer.Client/Models/AppBaseData.cs

@@ -0,0 +1,21 @@
+using System;
+using Newtonsoft.Json;
+
+namespace BTCPayServer.Client.Models;
+
+public class AppBaseData
+{
+    public string Id { get; set; }
+    public string AppType { get; set; }
+    public string AppName { get; set; }
+    public string StoreId { get; set; }
+    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
+    public bool? Archived { get; set; }
+    [JsonConverter(typeof(NBitcoin.JsonConverters.DateTimeToUnixTimeConverter))]
+    public DateTimeOffset Created { get; set; }
+}
+
+public interface IAppRequest
+{
+    public string AppName { get; set; }
+}