Refactor transcode_dicom_to_htj2k to use iterable interface

- transcode_dicom_to_htj2k now accepts file_loader (Iterable) instead of input_dir/output_dir
- Add DicomFileLoader class for simple file discovery and batching
- DicomFileLoader preserves directory structure in output paths
- Support for PyTorch DataLoader and any custom iterable
- Add proper error handling for files without PixelData in both nvimgcodec and pydicom paths
- Files causing exceptions during frame extraction are now properly skipped
- Add test demonstrating PyTorch DataLoader compatibility

Signed-off-by: Joaquin Anton Guirao <janton@nvidia.com>

Author: jantonguirao