Spike: Investigate further simplifications to the payment method definition/registration process

[brettshumaker]

Description

In #10217, we will merge (or "have merged" depending on when you're reading this) a mechanism to be able to register a payment method in WooPayments using a definition file. The goal is to simplify the process of adding/removing payment methods to the plugin. While the mechanism is functional, we think there might be another level of simplification we can add to further improve the payment method definition process.

Let's do a spike to investigate the possibility of doing away with the individual payment method classes and only passing the payment method definitions into the UPE_Payment_Method constructor. Keep in mind that we'll be gradually converting existing payment methods so any change will need to support both use cases until all payment methods have been converted.

Additional context

The comment that sparked this spike issue can be found here.