Allow repo add to set authentication for private repository

[Bao0ne]

Hi! Luet can access private repository by manually editing configuration files.I hope repo add can directly configure the authentication.

[jimmykarily]

I wonder if it would be better if we read the docker authentication from the regular docker config (e.g. Using Load and Dir) but I can't decide if we gain anything by that or not. Let's do it this way for now.

Just a small nit, the rest looks good.

[mudler]

Thank you!