Adressing the "view password" feature implementation- Issue #577

[ArisOIKON]

Summary

Added protectWithViewPassword(String password) to Worksheet,
allowing users to hide a sheet and lock the workbook structure
with a password so unauthorized users cannot unhide it.

How it works

• Worksheet.protectWithViewPassword(password) hides the sheet
  by setting its visibility state to HIDDEN and calls
  Workbook.protectStructure(password) to lock the workbook structure.

• Workbook.protectStructure(password) hashes the password and writes
  <workbookProtection workbookPassword="..." lockStructure="1"/>
  to workbook.xml.

Usage

java Worksheet ws = wb.newWorksheet("SecretSheet"); ws.value(0, 0, "Sensitive Data"); ws.protectWithViewPassword("myPassword");

Testing

• WorkbookProtectionTest: verifies workbook structure locking
• ViewPasswordE2ETest: verifies full round-trip: sheet
  visibility, data preservation and combined view/edit protection
• All test failures shown in the PR CI pipeline are pre-existing

As is the case with the pre-existing password, this is not full file encryption.
It uses Excel's built-in sheet hiding and workbook structure protection, which is meant to
discourage casual access rather than provide strong security.

[coveralls]

coverage: 88.791% (+0.07%) from 88.719% — ArisOIKON:master into dhatim:master

[ochedru]

Thank you for your contribution! Could you add a note to the README about this new feature? Thanks!