[react-centra-checkout]: drop CentraCheckout script and roll our own

[alexanderflink]

We are currently experiencing issues on a client project because it seems the CentraCheckout script is firing the Ingrid shipwallet_shipping_option_changed event incorrectly when the user presses the pay button in KCO, and then CentraCheckout script is calling Klarnas api.suspend() method, which hangs the KCO widget. This would not have been an issue if we simply handled these events and suspend / resume calls in our react-centra-checkout package. I also think it is a good thing in general to keep this kind of business critical logic on our end and not relying on a script tag from Centra. That way we have control over it and can easily debug it if something is wrong.

I suggest we start out by simply reading their checkout script carefully and re-implementing the logic (and removing aforementioned suspend call) while reading Klarna and Ingrid documentation to make sure everything is handled correctly. We could then release a (major?) version of this package and incrementally roll it out across client projects while monitoring for issues.