Make Ralph more extensible by implementing plugins support

[jmaupetit]

Purpose

Since the 3.0 release, Ralph has many optional dependencies depending on its usage: library (with backends support), CLI or LRS. It's not obvious to make dependency-management straightforward both for project developers and end-users.

Proposal

To simplify the various flavors of the project, as suggested by @sampaccoud in #218, we propose to implement a plugin management system that will simplify project maintenance and extensibility!

References:

1. https://packaging.python.org/en/latest/guides/creating-and-discovering-plugins/
2. https://setuptools.pypa.io/en/latest/userguide/entry_point.html

[sampaccoud]

thanks @ewjoachim for the idea on raincoat 🤭