Start saving a payment method

Begins saving a card for a contact. Returns a Stripe SetupIntent `clientSecret` to confirm client-side with Stripe Elements (the card never touches our servers). After the client confirms, call the attach endpoint with the returned `setupIntentId` to persist the card.

curl -X POST "https://api.3common.com/v1/contacts/string/payment-methods/setup-intent" \
  -H "Authorization: Bearer <token>"

{
  "data": {
    "setupIntentId": "string",
    "clientSecret": "string",
    "customerId": "string"
  }
}

export interface Response {
  data: {
    setupIntentId: string;
    clientSecret: string;
    customerId: string;
  };
}