[PM-3892] Implement dollar threshold for all subscriptions (#3283)

* Initial commit

* Fix the failing text

* Fix the unpaid invoice issue

* fix the unpaid invoice issue

* Changes for the threshold amount

* remove the billing threshold

* Add some comments to the old method

* Fixing issues on secret manager test

* import missing package

* Resolve pr comments

* Refactor PreviewUpcomingInvoiceAndPayAsync method

* Resolve some pr comments

* Resolving the comment around constant

* Resolve pr comment

* Add new class

* Resolve pr comments

* Change the prorateThreshold from 5 to 500 dollars

* Fix the failing test

* Fix the server returns a 500 error with the banner

src/Core/Constants.cs

@@ -24,6 +24,20 @@ public static class Constants
     public const string Fido2KeyCipherMinimumVersion = "2023.10.0";
 
     public const string CipherKeyEncryptionMinimumVersion = "2023.9.2";
+
+    /// <summary>
+    /// When you set the ProrationBehavior to create_prorations,
+    /// Stripe will automatically create prorations for any changes made to the subscription,
+    /// such as changing the plan, adding or removing quantities, or applying discounts.
+    /// </summary>
+    public const string CreateProrations = "create_prorations";
+
+    /// <summary>
+    /// When you set the ProrationBehavior to always_invoice,
+    /// Stripe will always generate an invoice when a subscription update occurs,
+    /// regardless of whether there is a proration or not.
+    /// </summary>
+    public const string AlwaysInvoice = "always_invoice";
 }
 
 public static class TokenPurposes