Refactor HEIC metadata extraction with extensible path system

- Separate EXIF tag IDs from root paths for better maintainability
- Create IFD_ROOT_PATHS, EXIF_ROOT_PATHS, GPS_ROOT_PATHS vectors
- Add ReadDateTimeFromPaths, ReadStringFromPaths, ReadIntegerFromPaths,
  ReadDoubleFromPaths, ReadMetadataFromPaths helper methods
- Refactor ExtractAllEXIFFields and ExtractGPSData to use path vectors
- Add HEIC/HEIF unit tests with graceful skip when test files unavailable

This refactoring follows DRY principles and makes it easy to add support
for additional image formats (PNG, WebP) in the future by simply adding
new root paths to the vectors.

Author: ThanhNguyxn