Payment methods' configuration UIs have been split off to a separate plugin type

Payment method plugins no longer offer functionality to configure them through the administrative UI. Instead, use payment method configuration plugins. To automatically expose payment method plugins based on payment method configuration plugins, use a deriver to provide derivative payment method plugin definitions.

Reasons for this change:

• Sometimes you want to write a payment method for which no UI or multiple instances are needed.
• Sometimes you will want a single configuration to be available as multiple different payment methods. This can be the case when a PSP (Payment Service Provider) has a single API through which multiple payment method brands, such as credit cards and wire transfers, are offered. By using a single payment method configuration plugin and letting the deriver create multiple payment method definitions of the configuration, you will only have to enter API credentials once, but customers will see the multiple brands as different payment methods.
• You can use the same approach to automatically expose payment methods based on configuration from another system (so not the current Drupal application).