API - Provide a way to check coupon eligibility#30

It would be nice if the API had a way of testing the coupon eligibility of a customer/product before making a checkout session. Right now, if a coupon is applied to checkout but none of the products are eligible (or the customer is ineligible), then nothing happens. The price just stays at the original price with no feedback.

It would be nice for headless integrations to test the coupon eligibility before so that we can provide feedback prior to redirecting to the checkout page.