Set a user agent when communicating with luarocks.org

Would be nice to have a user agent when talking to luarocks.org so we can see adoption, versions etc. The luarocks cli currently sends something like `LuaRocks/3.11.0 linux-x86_64 via LuaSocket` Minimally `lux/VERSION` would be helpful. I didn't see anything in this repo already setting the user agent Thanks