[14.x] Improve guest Checkout flow

[driesvints]

These changes are a followup to the guest checkout PR here: #1438. These allow for a more richer Checkout experience and a more fluent one. You can now use any method from the AllowsCoupons and HandlesTaxes traits:

$checkout = Checkout::guest()
    ->withCoupon('coupon_id')
    ->withTaxAddress('US', '72201', 'Arkansas')
    ->create();

Furthermore, I've managed to bring all of resolving of the Checkout Session options into the new CheckoutBuilder class which will help in the future when adding new features to the Cashier Stripe Checkout experience.

One thing I needed to change are the property visibilities on the AllowsCoupons and HandlesTaxes traits so I could more easily copy them over.

A future change I'd like to make is to move the contents of Checkout::create over to CheckoutBuilder::create but we can do that maybe in the next major release.