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/index.ts

@@ -1,62 +1,49 @@
-import type { Config } from 'payload'
 import { createInvoicesCollection, createPaymentsCollection, createRefundsCollection } from '@/collections'
 import type { BillingPluginConfig } from '@/plugin/config'
+import type { Config, Payload } from 'payload'
+import { createSingleton } from '@/plugin/singleton'
+import type { PaymentProvider } from '@/providers'
+
+const singleton = createSingleton(Symbol('billingPlugin'))
+
+type BillingPlugin = {
+  config: BillingPluginConfig
+  providerConfig: {
+    [key: string]: PaymentProvider
+  }
+}
+
+export const useBillingPlugin = (payload: Payload) => singleton.get(payload) as BillingPlugin
 
 export const billingPlugin = (pluginConfig: BillingPluginConfig = {}) => (config: Config): Config => {
   if (pluginConfig.disabled) {
     return config
   }
 
-  // Initialize collections
-  if (!config.collections) {
-    config.collections = []
-  }
-
-  config.collections.push(
+  config.collections = [
+    ...(config.collections || []),
     createPaymentsCollection(pluginConfig),
     createInvoicesCollection(pluginConfig),
     createRefundsCollection(pluginConfig),
-  )
+  ]
 
-  // Initialize endpoints
-  if (!config.endpoints) {
-    config.endpoints = []
-  }
-
-  config.endpoints?.push(
-    // Webhook endpoints
-    {
-      handler: (_req) => {
-        try {
-          const provider = null
-          if (!provider) {
-            return Response.json({ error: 'Provider not found' }, { status: 404 })
-          }
-
-          // TODO: Process webhook event and update database
-
-          return Response.json({ received: true })
-        } catch (error) {
-          // TODO: Use proper logger instead of console
-          // eslint-disable-next-line no-console
-          console.error('[BILLING] Webhook error:', error)
-          return Response.json({ error: 'Webhook processing failed' }, { status: 400 })
-        }
-      },
-      method: 'post',
-      path: '/billing/webhooks/:provider'
-    },
-  )
-
-  // Initialize providers and onInit hook
   const incomingOnInit = config.onInit
-
   config.onInit = async (payload) => {
-    // Execute any existing onInit functions first
     if (incomingOnInit) {
       await incomingOnInit(payload)
     }
-
+    singleton.set(payload, {
+      config: pluginConfig,
+      providerConfig: (pluginConfig.providers || []).reduce(
+        (acc, val) => {
+          acc[val.key] = val
+          return acc
+        },
+        {} as Record<string, PaymentProvider>
+      )
+    } satisfies BillingPlugin)
+    console.log('Billing plugin initialized', singleton.get(payload))
+    await Promise.all((pluginConfig.providers || []).map(p => p.onInit(payload)))
   }
 
   return config