Create a SetupIntent for saving a card with the inline payment element

Returns a `clientSecret` that Stripe's payment element exchanges for a saved card in the browser. The secret alone cannot charge anything or read existing cards, and it is scoped to the caller's own Stripe customer.

curl -X POST "https://example.com/api/me/billing/payment-method"

{  "clientSecret": "string",  "publishableKey": "string"}