Allow cugraph_service extensions to support more than graph creation

[rlratzel]

cugraph_service extensions currently only support graph creation, and make an assumption that they'll always return graph objects which can be referenced as graph IDs by clients. However, other operations can benefit from executing completely server-side, and extensions should allow for these use cases.

Graph creation extensions don't need to change, but additional extension types should be added which don't assume graph objects are returned. Other considerations:

• extensions which return data to the client (instead of just IDs)
• GPU-GPU data xfer

cc @VibhuJawa