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 by all different bundles to retrieve (non-abstract) entity or document classes defined in configuration.

To see an example, you can see what is done in my PR for SonataNewsBundle: https://github.com/sonata-project/SonataNewsBundle/pull/178/files#diff-649bcec1d33504d8c28a7d9ad16ae590R130