feat: enhance DRep certificate handling and testing
- Introduced `applyDRepCert` function to streamline DRep certificate operations (register, update, retire) in `buildDRepCertTx.ts`.
- Updated `RegisterDRep`, `Retire`, and `UpdateDRep` components to utilize the new `applyDRepCert` function for better code reuse and clarity.
- Added Jest tests for DRep certificate functionality in `drepCert.test.ts`, covering various scenarios including registration, update, and retirement.
- Created utility functions and fixtures for testing in `cborUtils.ts`, `fixtures.ts`, and `mockProvider.ts`.
- Established a GitHub Actions workflow for running unit tests and uploading coverage reports.
- Set coverage thresholds for critical files to ensure code quality.