[AC-1904] Implement endpoint to retrieve Provider subscription (#3921)

* Refactor Core.Billing prior to adding new logic

* Add ProviderBillingQueries.GetSubscriptionData

* Add ProviderBillingController.GetSubscriptionAsync

src/Core/Billing/BillingException.cs

@@ -0,0 +1,9 @@
+namespace Bit.Core.Billing;
+
+public class BillingException(
+    string clientFriendlyMessage,
+    string internalMessage = null,
+    Exception innerException = null) : Exception(internalMessage, innerException)
+{
+    public string ClientFriendlyMessage { get; set; } = clientFriendlyMessage;
+}