* changes to implement the email
* Refactoring and fix the unit testing
* refactor the code and remove used method
* Fix the failing test
* Update the email templates
* remove the extra space here
* Refactor the descriptions
* Fix the wrong subject header
* Add the in the hyperlink rather than just Help center
* 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
