Building off the foundation of using the Prices API, this PR adds the ability to create a one-time purchase plan for any product, which then can add a user to the specified plan group.
Some things to be aware of:
One-time purchases cannot have trials.
One-time purchases use the Invoice API instead of Subscriptions. Invoices are created then charged immediately.
Users should receive emails for these invoices directly from Stripe just like subscriptions.
Stripe has a newer API called Prices where you can create a price for any product and it can either be recurring or one-time. The easy part is existing Plans work with the Prices API by passing a Plan ID, but objects are returned in the slightly-different Prices API object format.
This commit is a refactor to the new API to handle the data in its new form, and lays the foundation for a one time payment plan to be added to any subscriptions product.
Justin DiRose