Add beforeSend hook for email customization

- Add BeforeSendHook type and BeforeSendMailOptions interface
- Implement hook execution in MailingService before sending emails
- Hook allows adding attachments, headers, and modifying email options
- Add comprehensive documentation with examples
- Bump version to 0.1.20

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-mailing",
-  "version": "0.1.19",
+  "version": "0.1.20",
   "description": "Template-based email system with scheduling and job processing for PayloadCMS",
   "type": "module",
   "main": "dist/index.js",