Implement Spreedly universal tokenization to vault a card once and charge it across multiple gateways

Steps

1. Set up a Spreedly environment and obtain your environment key and access secret from the Dashboard.
2. Collect the customer's card details using Spreedly's iFrame tokenization form or the API endpoint `POST /v1/payment_methods` (server-to-server, from a PCI-compliant server); Spreedly returns a payment method token.
3. Provision at least two gateways in your environment via `POST /v1/gateways` with the respective credentials; each gateway receives a unique Spreedly gateway token.
4. Execute a purchase or authorization against the primary gateway: `POST /v1/gateways/{gateway_token}/purchase` with `payment_method_token`; inspect `transaction.succeeded` and `transaction.response` in the response.
5. To charge the same card on a different gateway (for example, for a retry or a different market), repeat the purchase call using the same `payment_method_token` with a different `gateway_token`; no re-collection of card data is required.
6. For recurring or stored-credential transactions, include the `stored_credential` object with `initiator`, `reason_type`, and `network_transaction_id` from the initial CIT to comply with card network MIT requirements.