Add a CheckoutCompletedEvent payload to the onCheckoutCompleted callback

[kiftio]

What are you trying to accomplish?

Adds an overloaded onCheckoutCompleted(checkoutCompletedEvent: CheckoutCompletedEvent) to the EventProcessor interface and deprecates the no-arg onCheckoutCompleted().

The goal is to provide clients with more information about the checkout that just completed.

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Supporting sample app updates:

MobileBuyIntegration

• Updates logging of checkoutCompleted callbacks to record the payload
• Updates logs view to display recorded payload

SimpleCheckout

• Uses DefaultEventProcessor to avoid having to implement the deprecated no-arg onCheckoutCompleted()

Before you deploy

• [x] I have added tests to support my implementation
• [x] I have read and agree with the contributing documentation
• [x] I have read and agree with the code of conduct documentation
• [x] I have updated any documentation related to these changes.
• [x] I have updated the README (if applicable).