Add TimestampConverter

Author: solid-maksymtielnyi

lib/messages.g.dart

@@ -52,6 +52,7 @@ class CreatePreviewInvoiceSubscriptionDetailsRequest {
   /// invoice, and, for plans with month or year intervals, the day of the month
   /// for subsequent invoices. For existing subscriptions, the value can only be
   /// set to now or unchanged.
+  @TimestampConverter()
   final DateTime? billingCycleAnchor;
 
   /// A timestamp at which the subscription should cancel. If set to a date

lib/src/messages/requests/create_invoice_preview_request.dart

@@ -47,6 +47,7 @@ class CreateSubscriptionRequest {
   /// cycle. The anchor is the reference point that aligns future billing cycle
   /// dates. It sets the day of week for week intervals, the day of month for
   /// month and year intervals, and the month of year for year intervals.
+  @TimestampConverter()
   final DateTime? billingCycleAnchor;
 
   /// Mutually exclusive with billing_cycle_anchor and only valid with monthly