Add required apis to serialize the EntityPart

At the moment there is no api to get the object which is added to the EntityPart to serialize the entity to output stream.
It also needs getType() and getGenericType() to select the proper MessageBodyWriter.

Start by reading the EntityPart API and the issue’s requirements for exposing the contained object, its type, and its generic type. Determine how these values are needed by MessageBodyWriter selection; the work is done when EntityPart provides the required APIs and their behavior is covered by the project’s tests.

Written by the indexing model from the issue text.