* [NO LOGIC] [PM-21104] Organize Core.Billing tax code
* Add PreviewTaxAmountCommand and expose through TaxController
* Add PreviewTaxAmountCommandTests
* Run dotnet format
* Add PremiumUserSale
* Add PremiumUserBillingService
* Integrate into UserService behind FF
* Update invoice.created handler to bill newly created PayPal customers
* Run dotnet format