Refactor certificate management in certy.go

- Introduced helper functions for file path management, JSON reading/writing, and directory creation.
- Updated the `IssueCert` method to handle errors more gracefully and ensure proper account key management.
- Improved challenge token retrieval and error handling in the HTTP handler.
- Streamlined the certificate issuance process with better error reporting and state management.

This refactor enhances code readability and maintainability while ensuring robust error handling throughout the certificate management workflow.

Author: mberbero