[Plugin Protocol] Make BaseBuildPlugin generic with the Context Type

[JulienArzul]

What?

Since we now know the context_type, this PR actually uses it in the Plugin's protocol, so that all the methods can return the right type.

e.g: the declared context_type is now the type returned by build_context

[JulienArzul]

The change is trying to help the plugins type themselves better and avoid errors due to Any. But in the end, it looks like mypy ignores that generic type in some cases... So I'm not sure how interesting it is to declare (it makes the classes and especially the class hierarchy a bit more complex...