feat: only get datamaps from file

[maqi]

1. Created a new public helper function: streaming_encrypt_datamaps_from_file
   This function performs the core encryption logic but returns both the original and shrunk data maps without storing any chunks. It's memory-efficient and useful for scenarios where you need the data maps for analysis or want to handle chunk storage separately.
   Key features:

• Returns a tuple (DataMap, DataMap) - original map and shrunk map
• Uses placeholder chunk store functions to avoid memory overhead
• Maintains the same encryption logic as the original function
• Includes comprehensive documentation with examples

2. Refactored the original streaming_encrypt_from_file function
   The original function now uses a shared core implementation (streaming_encrypt_core) that can optionally store chunks. This eliminates code duplication while maintaining the same public API and functionality.
3. Added comprehensive unit tests

[dirvine]

Nice work @maqi I do feel we need an API overhaul in this crate with perhaps a new cleaner API and deprecate old API over time.