Add function to download and decrypt Text-AI prerelease

[tkilias]

We are distributing the prerelease of Text-AI using encrypted zip-files, and we need to download and decrypt them before able to install them.
The URL and the password are supplied via Environment Variables. We should provide a function which downloads the zipfile and decrypt it using the environment variables.

echo 'password' > unzip file.zip
``
You call unzip as a subprocess and need to supply the password via standard in.