[14.x] Provide findSetupIntent method

[andycowan]

The new Payment element requires a redirect to a new page after submitting the payment details, to enable the insertion of intermediate paymentMethod steps (for example bank pages etc.)

When the return_url is reaches, the setupIntent is supplied as a parameter, with the purpose of allowing the code to retrieve the setupIntent, to get the payment_method id.

This PR provides a findSetupIntent method on the billable entity, to enable retrieval of the setupIntent.

Test included.

This is described by @quantumwebco in issue #1491

fixes #1491

[driesvints]

Thanks!