Add encryption support for xls and xlsx / xlsm files

[manisocgen]

Hello NPOI Community,

Hope you’re having a great day!

Currently, NPOI does not support password protection for .xls and .xlsx files.

The below pull request enables encryption for both legacy Excel formats (.xls) and modern standards (.xlsx/.xlsm), ensuring compatibility with Microsoft Excel and other major spreadsheet tools.

The implementation follows the guidelines provided at Apache POI Encryption https://poi.apache.org/encryption.htm and is based on the Apache POI codebase. All related test cases have been successfully validated within NPOI.

I kindly request you to review below pull request and consider making the encryption feature available in NPOI.

Pull Request => #1659

Thank you!