Register payment methods via action hook (#254)

Replace the large, hardcoded payment method registration block in Plugin::register_payment_methods() with a do_action('pronamic_pay_register_payment_methods', $this->payment_methods) call. Remove the ImageService import and delegation of default method registrations so integrations and external packages can register payment methods in plugin space. Update CHANGELOG.md to document the new action, the changed registration flow, the move of Pronamic default methods to the pronamic-pay-default-payment-methods package, and a fix for Billink and Focum image assignment.

Author: remcotolsma

CHANGELOG.md

@@ -6,6 +6,19 @@ This projects adheres to [Semantic Versioning](http://semver.org/) and [Keep a C
 
 ## [Unreleased][unreleased]
 
+### Added
+
+- Added the `pronamic_pay_register_payment_methods` action to register payment methods in plugin space.
+
+### Changed
+
+- Updated payment method registration flow so payment methods are registered via the `pronamic_pay_register_payment_methods` action.
+- Moved Pronamic default payment method registration out of core and into the [`pronamic/pronamic-pay-default-payment-methods`](https://github.com/pronamic/pronamic-pay-default-payment-methods) package.
+
+### Fixed
+
+- Fixed image assignment for Billink and Focum in default payment method registration (now in [`pronamic/pronamic-pay-default-payment-methods`](https://github.com/pronamic/pronamic-pay-default-payment-methods)).
+
 ## [4.32.0] - 2026-03-23
 
 ### Added