[AC-1800] PayPal IPN Refactor (#3619)

* Add more logging to PayPal IPN webhook

* Add PayPalIPNClient tests

* Add PayPalControllerTests

---------

Co-authored-by: aelinton <95626935+aelinton@users.noreply.github.com>

test/Billing.Test/Utilities/PayPalTestIPN.cs

@@ -0,0 +1,37 @@
+namespace Bit.Billing.Test.Utilities;
+
+public enum IPNBody
+{
+    SuccessfulPayment,
+    ECheckPayment,
+    TransactionMissingEntityIds,
+    NonUSDPayment,
+    SuccessfulPaymentForOrganizationCredit,
+    UnsupportedTransactionType,
+    SuccessfulRefund,
+    RefundMissingParentTransaction,
+    SuccessfulPaymentForUserCredit
+}
+
+public static class PayPalTestIPN
+{
+    public static async Task<string> GetAsync(IPNBody ipnBody)
+    {
+        var fileName = ipnBody switch
+        {
+            IPNBody.ECheckPayment => "echeck-payment.txt",
+            IPNBody.NonUSDPayment => "non-usd-payment.txt",
+            IPNBody.RefundMissingParentTransaction => "refund-missing-parent-transaction.txt",
+            IPNBody.SuccessfulPayment => "successful-payment.txt",
+            IPNBody.SuccessfulPaymentForOrganizationCredit => "successful-payment-org-credit.txt",
+            IPNBody.SuccessfulRefund => "successful-refund.txt",
+            IPNBody.SuccessfulPaymentForUserCredit => "successful-payment-user-credit.txt",
+            IPNBody.TransactionMissingEntityIds => "transaction-missing-entity-ids.txt",
+            IPNBody.UnsupportedTransactionType => "unsupported-transaction-type.txt"
+        };
+
+        var content = await EmbeddedResourceReader.ReadAsync("IPN", fileName);
+
+        return content.Replace("\n", string.Empty);
+    }
+}