Feature: support for policy plugins in sudo-plugin

[nbraud]

sudo-plugin, despite its generic name, currently only supports creating IO plugins, and not policy plugins. Would you consider policy plugins to be in-scope for the crate?

PS: Many thanks for releasing sudo-pair, and making sudo-plugin available as a reusable crate.

[stouset]

Yep! I just haven't done the work, but I would love for that to be added.