iap pre-purchase check

src/Core/Models/Api/Request/IapCheckRequestModel.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using Bit.Core.Enums;
+
+namespace Bit.Core.Models.Api
+{
+    public class IapCheckRequestModel : IValidatableObject
+    {
+        [Required]
+        public PaymentMethodType? PaymentMethodType { get; set; }
+
+        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
+        {
+            if(PaymentMethodType != Enums.PaymentMethodType.AppleInApp)
+            {
+                yield return new ValidationResult("Not a supported in-app purchase payment method.",
+                    new string[] { nameof(PaymentMethodType) });
+            }
+        }
+    }
+}

src/Core/Models/Business/BillingInfo.cs

@@ -15,6 +15,8 @@ namespace Bit.Core.Models.Business
 
         public class BillingSource
         {
+            public BillingSource() { }
+
             public BillingSource(PaymentMethod method)
             {
                 if(method.Card != null)