Detach payment method associated to a setup intent on guest checkout

[waiting-for-dev]

When performing a guest checkout through the setup intent flow, creating another payment from the Stripe dashboard is possible.

A solution via a subscriber was attempted as part of #203. The code looked like the following, although it'd need refining after the final version is released:

# frozen-string-literal: true

module SolidusStripe
  class SetupIntentSubscriber
    include Omnes::Subscriber

    handle :order_finalized, with: :detach_setup_intent

    def detach_setup_intent(event)
      order = event[:order]
      payment_method = order.payments.find do |payment|
        payment.payment_method.type == "SolidusStripe::PaymentMethod"
      end&.payment_method
      return unless payment_method && payment_method.preferred_stripe_intents_flow == "setup" && order.user.nil?

      intent = payment_method.find_setup_intent_for_order(order)
      payment_method.gateway.request do
        Stripe::PaymentMethod.detach(intent.payment_method)
      end
    end
  end
end

Some considerations:

• Detaching a payment method from Stripe is the API call we need. We could probably also cancel the setup intent after that: it can't be canceled before because of having an associated payment method.
• We can't use the callback controller, as that's leaving the order in the confirm state. Later, on order.complete!, we still need access to the payment method to authorize the payment.
• Because of the above, a possible solution is the subscriber that runs when the order is finalized.
• The previously created payment intent can still be manually captured after the payment method is detached.
• Running on the :order_finalized event has the known issue of blocking the database transaction.

[elia]

Not needed since we're moving to only use PaymentIntents in #249