[PM-21881] Manage payment details outside of checkout (#6032)

* Add feature flag

* Further establish billing command pattern and use in PreviewTaxAmountCommand

* Add billing address models/commands/queries/tests

* Update TypeReadingJsonConverter to account for new union types

* Add payment method models/commands/queries/tests

* Add credit models/commands/queries/tests

* Add command/query registrations

* Add new endpoints to support new command model and payment functionality

* Run dotnet format

* Add InjectUserAttribute for easier AccountBillilngVNextController handling

* Add InjectOrganizationAttribute for easier OrganizationBillingVNextController handling

* Add InjectProviderAttribute for easier ProviderBillingVNextController handling

* Add XML documentation for billing command pipeline

* Fix StripeConstants post-nullability

* More nullability cleanup

* Run dotnet format

src/Api/Utilities/StringMatchesAttribute.cs

@@ -0,0 +1,18 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Bit.Api.Utilities;
+
+public class StringMatchesAttribute(params string[]? accepted) : ValidationAttribute
+{
+    public override bool IsValid(object? value)
+    {
+        if (value is not string str ||
+            accepted == null ||
+            accepted.Length == 0)
+        {
+            return false;
+        }
+
+        return accepted.Contains(str);
+    }
+}