Add request body param converter for APIs

In order to use serializations which are mapped on abstract models, I've added this service.

This service will be used in all different bundles to retrieve configured (non-abstract) entity or document classes, which that is done in my PR for SonataNewsBundle, see:

https://github.com/sonata-project/SonataNewsBundle/pull/178/files#diff-649bcec1d33504d8c28a7d9ad16ae590R130