[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

test/Core.Test/Billing/Payment/Queries/GetCreditQueryTests.cs

@@ -0,0 +1,41 @@
+using Bit.Core.Billing.Payment.Queries;
+using Bit.Core.Billing.Services;
+using Bit.Core.Entities;
+using NSubstitute;
+using NSubstitute.ReturnsExtensions;
+using Stripe;
+using Xunit;
+
+namespace Bit.Core.Test.Billing.Payment.Queries;
+
+public class GetCreditQueryTests
+{
+    private readonly ISubscriberService _subscriberService = Substitute.For<ISubscriberService>();
+    private readonly GetCreditQuery _query;
+
+    public GetCreditQueryTests()
+    {
+        _query = new GetCreditQuery(_subscriberService);
+    }
+
+    [Fact]
+    public async Task Run_NoCustomer_ReturnsNull()
+    {
+        _subscriberService.GetCustomer(Arg.Any<ISubscriber>()).ReturnsNull();
+
+        var credit = await _query.Run(Substitute.For<ISubscriber>());
+
+        Assert.Null(credit);
+    }
+
+    [Fact]
+    public async Task Run_ReturnsCredit()
+    {
+        _subscriberService.GetCustomer(Arg.Any<ISubscriber>()).Returns(new Customer { Balance = -1000 });
+
+        var credit = await _query.Run(Substitute.For<ISubscriber>());
+
+        Assert.NotNull(credit);
+        Assert.Equal(10M, credit);
+    }
+}