v1.0.0: Add WP S3 Sync plugin initial import

Add initial WP S3 Sync plugin implementation. Includes plugin entry (wp-s3-sync.php), README, admin assets (CSS/JS), and core classes: main plugin class, settings page, S3 client (AWS Signature V4 implemented with cURL), uploader, URL rewriter, and bulk migration handler with AJAX. Adds direct upload integration, presigned URL/CORS helpers, three upload modes (sync/offload/direct), and UI for testing connection and bulk migrating media.