Cashier-stripe: Better coupon support & promotion codes

[driesvints]

Add some better APIs to work with coupons on subscriptions, etc so people don't need to do this themselves with the Stripe SDK. Add a specific Coupon object.

Things to consider:

• Coupon object
• Validate coupons
• Retrieve coupon code from subscriptions
• Update coupon code on subscriptions

[ReckeDJ]

Don't forget the (new) Promotion Codes feature!
https://stripe.com/docs/billing/subscriptions/discounts/codes

[driesvints]

@ReckeDJ been looking into that today a bit as part of #1007. Think we can add some simple methods for that as well.

[driesvints]

I already sent in a small PR in advance for this: https://github.com/laravel/cashier-stripe/pull/1032