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/index.ts

@@ -0,0 +1,2 @@
+export * from './mollie'
+export * from './types'

src/providers/mollie.ts

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+import type { Payment } from '@/plugin/types/payments'
+import type { InitPayment, PaymentProvider } from '@/plugin/types'
+import type { Payload } from 'payload'
+import { createSingleton } from '@/plugin/singleton'
+import type { createMollieClient, MollieClient } from '@mollie/api-client'
+
+const symbol = Symbol('mollie')
+export type MollieProviderConfig = Parameters<typeof createMollieClient>[0]
+
+export const mollieProvider = (config: MollieProviderConfig) => {
+  const singleton = createSingleton<MollieClient>(symbol)
+  return {
+    key: 'mollie',
+    onInit: async (payload: Payload) => {
+      const createMollieClient = (await import('@mollie/api-client')).default
+      const mollieClient = createMollieClient(config)
+      singleton.set(payload, mollieClient)
+    },
+    initPayment: async (payload, payment) => {
+      if (!payment.amount) {
+        throw new Error('Amount is required')
+      }
+      if (!payment.currency) {
+        throw new Error('Currency is required')
+      }
+      const molliePayment = await singleton.get(payload).payments.create({
+        amount: {
+          value: (payment.amount / 100).toFixed(2),
+          currency: payment.currency
+        },
+        description: payment.description || '',
+        redirectUrl: 'https://localhost:3000/payment/success',
+        webhookUrl:  'https://localhost:3000',
+      });
+      payment.providerId = molliePayment.id
+      payment.providerData = molliePayment.toPlainObject()
+      return payment
+    },
+  } satisfies PaymentProvider
+}

src/providers/types.ts

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+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
+}