core: entity mapping - refactor how entities are deserialized from request body [BREAKING]

This follows up the changes introduced at response level, to make them symmetric.

The change is breaking at API level: StdEntityRoute constructor has changed, you should now use StdJsonProducerEntityRoute instead if you want the same behavior as before for routes only producing json, and not consuming it.
core: refactor: inline content type factories and default providers in modules

to minimize the number of top level classes cluttering the package structure