Improve data display on receipt (#5896)

Once more an improvement for the receipt, which also fixes #5882:

- Unify data displayed on the web and print version
- Split cart and additional data and ensure additional data is displayed
- Do not display extra subtotal row if there are no tips or discounts
- Make PosData partial more universal and backwards-compatible by using case insensitive key lookups

BTCPayServer/Models/InvoicingModels/InvoiceReceiptViewModel.cs

@@ -17,6 +17,7 @@ namespace BTCPayServer.Models.InvoicingModels
         public decimal Amount { get; set; }
         public DateTimeOffset Timestamp { get; set; }
         public Dictionary<string, object> AdditionalData { get; set; }
+        public Dictionary<string, object> CartData { get; set; }
         public ReceiptOptions ReceiptOptions { get; set; }
         public List<ViewPaymentRequestViewModel.PaymentRequestInvoicePayment> Payments { get; set; }
         public string OrderUrl { get; set; }