Support of extensability for Kinds

[ohault]

In a future major release of Containerlab, I would like to suggest to introduce a way to support extensability for [Kings] (https://containerlab.dev/manual/kinds/)

At first, I can think about a plugins style architecture where each Kings would become a plugin.
Additionnly we could also consider add a way to extend an existing King (plugin).

As use cases, I can think about using Kings for IoT devices/ NAS appliances / ...