Try to fix Simple pager

[VincentLanglet]

Subject

I am targeting this branch, because bugfix.
It could technically be considered as a BC-break since execute method is now returning a Paginator instead of an array. But the phpdoc were saying mixed so I don't think there is a real BC-promise about the returning value.
It's still Countable and Traversable, and this is the only solution I found.

Closes #1403.

Changelog

### Changed
- `ProxyQuery::execute()` is now returning a Paginator instead of an array.

### Fixed
- Support for fetch join with simple pager.

[VincentLanglet]

\count($this->getQuery()->execute()) is technically not always allowed.
Need some discussions about sonata-project/SonataAdminBundle#7112.

But @franmomu can you tried my fix on your project ?

[franmomu]

But @franmomu can you tried my fix on your project ?

👍 looks like it works fine

[VincentLanglet]

Please review @sonata-project/contributors

[VincentLanglet]

I also like to release this bugfix @dmaicher