FORGE-1060 Added a two-level DTO creator for the REST plugin

The plugin now allows creation of two-level DTOs (a root and a nested DTO) that are eventually used in the created JAX-RS resources. The DTOs prevent issues related to cyclic and bidirectional relationships in JPA object graphs, by terminating the cycle irrespective of it's presence.

Additionally, updates to collection-based associations can be performed by the client sending an updated collection with the Ids. Updates to other collection attributes are ignored at the moment.