add Doctrine MongoDB converter

[IonBazan]

This PR introduces Doctrine MongoDB converter. Since DoctrineParamConverter relies on Doctrine\Persistence\ManagerRegistry, it can consume any persistence engine so we can simply replace the argument to make it work same as ORM.

The only thing that is ORM-specific is the name of entity_manager option - it should be named object_manager but I decided to keep it as is for BC.