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/plugin/config.ts

@@ -1,5 +1,6 @@
 import { CollectionConfig } from 'payload'
 import { FieldsOverride } from '@/plugin/utils'
+import { PaymentProvider } from '@/plugin/types'
 
 export const defaults = {
   paymentsCollection: 'payments',
@@ -63,11 +64,7 @@ export interface BillingPluginConfig {
   customerInfoExtractor?: CustomerInfoExtractor // Callback to extract customer info from relationship
   customerRelationSlug?: string // Customer collection slug for relationship
   disabled?: boolean
-  providers?: {
-    mollie?: MollieConfig
-    stripe?: StripeConfig
-    test?: TestProviderConfig
-  }
+  providers?: PaymentProvider[]
   webhooks?: {
     basePath?: string
     cors?: boolean