* [PM-25088] add feature flag for new premium subscription flow
* [PM-25088] refactor premium endpoint
* forgot the punctuation change in the test
* [PM-25088] - pr feedback
* [PM-25088] - pr feedback round two
* 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