Drive - Automation Operations : better JSON serialization

Drive uses a set of dedicated Operations: these operations don't return a DocumentModel but the JSON serialization of the Adapter objects.

For now, the Operation :

• handles JSON serialization inside the code
• returns Blob type

This does make sense for 5.6, but with the ongoing work on Automation in 5.7, this is not good :

• serialization should be part of the JAX-RS/Automation marshaling infrastructure
• Operation signature should return the Adapter object and not a Blob