Support specifying payment method through apps (#1539)

* Support specifying payment method through apps

closes #1525
This is great if you want to offer items with an incentive to use a specific payment method without messing with the rates.
For example, you can have `Item A` which costs 25$ and your store is configured for USDT and BTC. You can create two items, with different prices, where Item A costs 20$ if you pay with bitcoin or 25$ if you paid in dirty fiat pegs

* make code cleaner

* Add Test

* fix test

* fix test

BTCPayServer/Models/AppViewModels/ViewPointOfSaleViewModel.cs

@@ -21,6 +21,7 @@ namespace BTCPayServer.Models.AppViewModels
             public string Title { get; set; }
             public bool Custom { get; set; }
             public int? Inventory { get; set; } = null;
+            public string[] PaymentMethods { get; set; }
         }
 
         public class CurrencyInfoData