Provide an easy way to debug "behind the scenes" fixture creation with hooks

[smarie]

Many functions in the pytest_cases API create fixtures behind the scenes:

• unpack_fixture
• fixture_union
• param_fixture[s]
• parametrize_plus

This is because the design philosophy is not to mess with pytest internals when possible.

However, users may get lost when something unexpected happens. Besides all these fixture functions created behind the scenes can not be easily decorated: one cannot apply for example pytest-harvest's @saved_fixture decorator.

For all these reasons and to ease future maintenance/refactoring/debugging process, I propose to allow all the above functions to receive a hook argument, that will be called everytime a fixture is about to be created.