feat: Add Mollie payment provider support

- Introduce `mollieProvider` for handling Mollie payments
- Add configurable payment hooks for initialization and processing
- Implement `initPayment` logic to create Mollie payments and update metadata
- Include types for Mollie integration in payments and refunds
- Update `package.json` to include `@mollie/api-client` dependency
- Refactor existing payment-related types into modular files for better maintainability

src/providers/types.ts

@@ -0,0 +1,10 @@
+import type { Payment } from '@/plugin/types/payments'
+import type { Payload } from 'payload'
+
+export type InitPayment = (payload: Payload, payment: Partial<Payment>) => Promise<Partial<Payment>>
+
+export type PaymentProvider = {
+  key: string
+  onInit: (payload: Payload) => Promise<void> | void
+  initPayment: InitPayment
+}