Allow to extract values from enum types at `AbstractPropertySourceIterator::getValue()`

[phansys]

Subject

Allow to extract values from enum types at AbstractPropertySourceIterator::getValue()

I am targeting this branch, because this change respects BC.

Changelog

### Added
- Support for enum types in exported values.

[phansys]

Thanks for you review @VincentLanglet. I just made a new push adding an option to allow users to control the behavior for backed enums, in order to choose the enum's name even when the enum is backed.