Link Stripe Payment Intents to Spree::Payment

[elia]

What

We need to map each Spree::Payment to a single Payment Intent, and thus have a single Stripe Spree::Payment for a given order that will track all the payment attempts made for the Payment Intent. Stripe recommends having a single payment intent per order.

Tasks

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1. connect the payment intent id to the payment transaction_id
2. implement the state mapping as described below

Dependencies

• blocks Add support for reusable payment sources #153

Mapping between Solidus and Stripe payment states

The following is the tentative mapping between the states in the two systems, since in Stripe the "failed" state is just going back to the starting point we'll treat anything before a final state as :processing.

Why

Since Stripe considers updates sent via webhooks to be the most reliable source of truth, we need a place in which to track the history of each payment tracking attempts, failures, refusals, successes etc.

The entity that can best take up this role seems to be Spree::Payment.