[WIP] poc/apiimport2.0

Yes I know but for me you've two solutions in this case:
- Add an encoder (kind of dummy one) to make it pass
- Make the Akeneo PIM Serializer pass useable without adding encoder

I hesitated to change the encoder as an optional value when I updated this compiler pass but I kept it to be homogeneous with Symfony one.
To discuss with @BitOne and @nidup but for me duplicate code is not an option especially on a connector