[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/Api/Billing/Models/Responses/ProviderSubscriptionResponse.cs

@@ -2,6 +2,7 @@
 using Bit.Core.AdminConsole.Enums.Provider;
 using Bit.Core.Billing.Enums;
 using Bit.Core.Billing.Models;
+using Bit.Core.Billing.Providers.Models;
 using Bit.Core.Billing.Tax.Models;
 using Stripe;