Maybe we can start with this, but disable it by default and label it a bit experimental. I.e. add `config.git_proxy.enabled` which is `false` by default. What do you think?

Then later we can change this to use `git config` when the infra is ready for hooking to provision actions. I'll need that for e.g. maven support anyway. And we can anyway enable this by default later. Other way around it might be a breaking change for somebody.