Support additional inputs to custom subgraph ops that are not direct dependencies to ops in the subgraph. This will enable various use cases: custom control flow ops, custom ops that maintain a state that should be saved/loaded, etc.

Highlights:

* Added test that uses a graph pass (addInputPass) to add a new custom input to the subgraph op

* Added new optional argument (clear) to hybridize & optimize_for APIs in Gluon Block to enable multiple optimizations

* refactored lib_api.h JSON utilities

* added new Graph data structure utilities to simplify custom graph passes

* refactored custom op registration

* enhanced custom subgraph op to support additional inputs to subgraph op that is not an input to ops in the subgraph

* updated subgraph & graph pass READMEs

* Added error messaging from external library

… subgraph_v18

… subgraph_v18