Entrypoints plugin

[nenb]

This is an initial attempt at adding support for entry points for plugins following #189.

Current status: Question marks over the implementation and whether it is truly needed. See #189 for latest details.

[pmeier]

@nenb I just released ragna-aws: https://github.com/pmeier/ragna-aws. I'm going to share a few pain points in form of issues soon. Forcing users to manually edit their ragna.toml to get the functionality they want is one of them. We can use the package as test bed for the entrypoints.