Add R2 Diagnostic Tool for 403 Error Troubleshooting

Commit notes

- Introduced a new R2 diagnostic tool to check configuration and troubleshoot common 403 errors. - Implemented checks for R2 account ID, access key, secret key, and bucket existence. - Added functionality to generate presigned URLs and test file operations. - Enhanced configuration validation in the config package for R2-specific settings, ensuring required fields are set when using R2 as the storage provider.