REST plugin should support user-driven mapping of DTOs to JPA entities

This is to extend the work done in FORGE-1060 which is scoped to generate DTOs in a pre-determined manner for JPA entities. We should be able to support user-driven mapping of the JPA entities to create the DTOs first, and then generate the appropriate business services that would update the underlying entities in the database.

Doing so would allow the REST resources to function in a similar manner to the REST workflow pattern. This would improve the transactional behavior of the resources, since the constructed DTOs could house all related entities that participate in a business transaction. Since metadata of business transactions is not available in the object model, the mapping process to derive the DTOs will be user-driven.

Not doing so would yield REST resources that are just representations of database tables, thus requiring associations to be established for entities that participate in a business transaction.