Define Checkout schema

[lfades]

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Our current checkout type is set to any and is defined here: framework/commerce/types/checkout.ts. We need to have it defined to a shape that can work with any e-commerce provider we throw at it and with third party tools like Stripe, as we'll support custom checkout implementations for the providers that either don't have one or allow you to customize it.

Every field defined needs to have docs, we do that using comments, e.g:

/**
 * The checkout's id.
 */
id: string