feature: add support for ssh key authentication

[greenpau]

A clear and concise description of what you want the system to do.

Add support for ssh key authentication.

Github docs: https://docs.github.com/en/authentication/connecting-to-github-with-ssh

What are the Caddyfile directives that need to be added.

Add Caddyfile directive:

git {
  repo foo {
    url git@github.com:greenpau/caddy-git.git
    auth key <path> [passphrase <secret>]
  }
}

[greenpau]

Added caddyfile directives in 3918564