[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/Models/MaskedPaymentMethodTests.cs

@@ -0,0 +1,63 @@
+using System.Text.Json;
+using Bit.Core.Billing.Payment.Models;
+using Xunit;
+
+namespace Bit.Core.Test.Billing.Payment.Models;
+
+public class MaskedPaymentMethodTests
+{
+    [Fact]
+    public void Write_Read_BankAccount_Succeeds()
+    {
+        MaskedPaymentMethod input = new MaskedBankAccount
+        {
+            BankName = "Chase",
+            Last4 = "9999",
+            Verified = true
+        };
+
+        var json = JsonSerializer.Serialize(input);
+
+        var output = JsonSerializer.Deserialize<MaskedPaymentMethod>(json);
+        Assert.NotNull(output);
+        Assert.True(output.IsT0);
+
+        Assert.Equivalent(input.AsT0, output.AsT0);
+    }
+
+    [Fact]
+    public void Write_Read_Card_Succeeds()
+    {
+        MaskedPaymentMethod input = new MaskedCard
+        {
+            Brand = "visa",
+            Last4 = "9999",
+            Expiration = "01/2028"
+        };
+
+        var json = JsonSerializer.Serialize(input);
+
+        var output = JsonSerializer.Deserialize<MaskedPaymentMethod>(json);
+        Assert.NotNull(output);
+        Assert.True(output.IsT1);
+
+        Assert.Equivalent(input.AsT1, output.AsT1);
+    }
+
+    [Fact]
+    public void Write_Read_PayPal_Succeeds()
+    {
+        MaskedPaymentMethod input = new MaskedPayPalAccount
+        {
+            Email = "paypal-user@gmail.com"
+        };
+
+        var json = JsonSerializer.Serialize(input);
+
+        var output = JsonSerializer.Deserialize<MaskedPaymentMethod>(json);
+        Assert.NotNull(output);
+        Assert.True(output.IsT2);
+
+        Assert.Equivalent(input.AsT2, output.AsT2);
+    }
+}