[PM-21099] (NO LOGIC) Organize Billing provider code (#5819)

* [NO LOGIC] Organize Billing provider code

* Run dotnet format

* Run dotnet format'

* Fixed using after merge

* Fixed test usings after merge

src/Core/Billing/Providers/Entities/ProviderInvoiceItem.cs

@@ -0,0 +1,34 @@
+#nullable enable
+
+using System.ComponentModel.DataAnnotations;
+using Bit.Core.Entities;
+using Bit.Core.Utilities;
+
+namespace Bit.Core.Billing.Providers.Entities;
+
+public class ProviderInvoiceItem : ITableObject<Guid>
+{
+    public Guid Id { get; set; }
+    public Guid ProviderId { get; set; }
+    [MaxLength(50)]
+    public string InvoiceId { get; set; } = null!;
+    [MaxLength(50)]
+    public string? InvoiceNumber { get; set; }
+    public Guid? ClientId { get; set; }
+    [MaxLength(50)]
+    public string ClientName { get; set; } = null!;
+    [MaxLength(50)]
+    public string PlanName { get; set; } = null!;
+    public int AssignedSeats { get; set; }
+    public int UsedSeats { get; set; }
+    public decimal Total { get; set; }
+    public DateTime Created { get; set; } = DateTime.UtcNow;
+
+    public void SetNewId()
+    {
+        if (Id == default)
+        {
+            Id = CoreHelpers.GenerateComb();
+        }
+    }
+}