Add private key by user support for git push (and any ssh related action)

In order to push to private git repository (not just public github) it could be interesting to have the ability to generate a private/public key pair that we can then link to our github/gitlab account (to avoid having to put our own private key on glitch which make no sens at all).
This will allow us to sign and to use ssh+git to push commit upward to the origin server.

The idea is to automatically create a .ssh/ folder with this private key on any instance that is created for any project.