[junit5] Extensions to handle multiple layers of scope

[kriegfrj]

It would be nice if our Jupiter extensions (eg) BundleContextExtension handled multiple layers of scope in a sensible way.

In a test class, for example, you might have a certain dummy service you wish to install for the life of the class, so that it is available for all tests in that class, and for all nested test classes in that class. Then give each individual method/nested class the ability to install services for the life of that test method/class, etc.

I know that Jupiter's extension model has been designed with this sort of hierarchy in mind. With a few tweaks we could probably leverage it in our extensions.