Only display storage providers that can actually be set in dropdown

BTCPayServer/Storage/ViewModels/ChooseStorageViewModel.cs

@@ -1,9 +1,12 @@
+using System.Collections.Generic;
 using BTCPayServer.Storage.Models;
+using Microsoft.AspNetCore.Mvc.Rendering;
 
 namespace BTCPayServer.Storage.ViewModels
 {
     public class ChooseStorageViewModel
     {
+        public IEnumerable<SelectListItem> ProvidersList { get; set; }
         public StorageProvider Provider { get; set; }
         public bool ShowChangeWarning { get; set; }
     }