[12.x] Implement webhook command

[driesvints]

This implements a webhook command to easily set up the webhook needed to send the events that Cashier needs from Stripe. By default it'll use the APP_URL with the default webhook route and the default Cashier version. Both of these can be customized. You can also pass a --disabled flag if you don't want it to start sending events straight away.

# Create the webhook with the events needed for Cashier
php artisan cashier:webhook

# Set an explicit url
php artisan cashier:webhook --url="http://example.com/stripe/webhook"

# Set an explicit Stripe API version
php artisan cashier:webhook --api-version="2019-12-03"

# Disable the webhook upon creation
php artisan cashier:webhook --disabled

This will make the setup process that much easier for new Cashier users and users that are upgrading to newer versions. We can add this command to the upgrade guide.