Unfortunately, Caddy does not support and do not plan on supporting anything other than HTTP/HTTPS. These days I find myself going back to nginx only for TCP/UDP reverse proxy.
It supports it with the caddy-l4 plugin: https://github.com/mholt/caddy-l4. It was also indicated we might move the plugin into standard Caddy once given enough feedback from the user-base and are comfortable with the implementation solidity.