Introduce mechanism to enable specific global extensions in JUnit Jupiter

[translatenix]

Enabling all global extensions on the class/module path with junit.jupiter.extensions.autodetection.enabled=true is asking for surprises (and potentially trouble).

Instead, I'd like to explicitly enable the global extensions that I want to use.

I imagine that I'd do this by setting junit.jupiter.extensions.autodetection.enabled=foo.bar.Baz,foo.bar.Qux or a similar property.

If the JUnit team is open to this idea, I'd be happy to discuss further and send a PR.