[IIDM v1.1] New extension design

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature
  Allow to have different implementations of extensions powsybl-core#1152

• What is the current behavior?
  In Java, we now support two different implementation of the IIDM model (transient and persistent).

• What is the expected behavior?
  Both Java and C++ implementation should be identical

• What is the motivation / use case for changing the behavior?
  Have both Java and C++ API as close as possible

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)