Read ID & SECRET from files

[thalin]

A standard security practice is to pass secret values in files so that they aren't exposed via /proc and/or ps. It would be great if you could specify --id-file and --secret-file to pass along these values as files instead of directly on the command line.

An additional bonus would be being able to set _FILE environment variables for these as well, to match the existing environment variables from which newt reads secrets now.

[oschwartz10612]

The id and secret are actually saved in the files at the following locations, depending on your operating system after you do a initial connection with the values. This should be documented better and we should also add a way to control the files so I will leave this open.

macOS: ~/Library/Application Support/newt-client
Windows: %APPDATA%\newt-client
Linux (and others): ~/.config/newt-client