Migrate to new Janus configuration style

[mqp]

Right now, the plugin reads its configuration in INI format, like Janus used to, using a Rust library that knows how to parse INIs. But Janus migrated everything else to libconfig-style configuration. So it would make a lot of sense to migrate the plugin configuration, too.

Janus exposes its config reading/writing API in C, so it's probably good to use the API it exposes rather than hunting down a separate Rust library that knows how to read libconfig syntax.