[Draft] read encrypted xlsx files

[JanMarvin]

Using a slightly modified version of https://github.com/maharmstone/xlcpp/ to decrypt password protected xlsx files. If a password is provided, we should be able to decrypt the xlsx file. The decrypted file is written to disk and imported.

I do not really like this

• Initially I was aiming to get https://github.com/tfussell/xlnt working. But while they have the cryptography files all sorted in a nice little folder, when I was building, I always ran into a wall. My issue is, that the crypto files are not simply to extract into an independent library.
• xlcpp is aiming for its own workbooks and I built the entire library, just for the decryption part, so this has to be cleaned out.
• It depends on libarchive (not sure this is needed) and C++20 features.

  library(openxlsx2)
  fl <- "https://github.com/JanMarvin/openxlsx-data/raw/main/openxlsx2_example_pass.xlsx"
  
  wb <- wb_load(fl, password = "openxlsx2")

[JanMarvin]

Ah whatever, you have to take my word for it that it builds :)

[JanMarvin]

Cleaned the library quite a bit. Builds on Windows, Linux and Mac (on my local one at least), but C++20 is to much

[JanMarvin]

Another solution that actually works really well is https://github.com/herumi/msoffice.

It needs openssl as SystemRequirement and if linked with a static library, encoding and decoding is possible.