[14.x] Add ability to ignore incomplete payments

[driesvints]

Right now, it's not possible to create subscriptions without also automatically confirming them. It would be handy for apps where the frontend like Next.js etc handles the confirmation through Stripe.js for example without providing a payment method up front.

$subscription = $user->newSubscription('default', 'price_xxx')
    ->ignoreIncompletePayment()
    ->create();

I'm not 100% sure about the naming of ignoreIncompletePayment yet.