Creates an unconfirmed, card-only setup intent for the account's payment customer and returns its id, client secret and the publishable key of the payment system account it was created in.

Adding a card is a three-step flow:

• Call this endpoint to create the setup intent.
• In the browser, initialise the payment system's client library with 'publishableKey', collect the card and confirm the setup intent with 'clientSecret'.
• Call the add payment method endpoint with 'setupIntentId' and 'clientSecret' as the payment information.

{
  "setupIntentId": "seti_1234567890",
  "clientSecret": "seti_1234567890_secret_5678901234",
  "publishableKey": "pk_test_1234567890"
}