Secret inputs

[ehmicky]

A plugin author (@brunoquaresma) have asked for the possibility to add secret inputs.

At the moment, Build plugins can do this using either:

• environment variables
• the filesystem (reading configuration files in the user's build directory)

However some systems like Travis provide the ability to encrypt values locally and add the encrypted value to the configuration file. GitHub actions on the other hand also uses environment variables, but they allow inserting directly in the workflow.yml using templating.

Are we good with our current solution? Should we consider alternatives?