Add an endpoint for 'scenegraph' to the server.

Processes requests using the scenegraph package:
https://nlp.stanford.edu/software/scenegraph-parser.shtml

Output is in either the text or json format from the scenegraph package.

Requested in https://github.com/stanfordnlp/CoreNLP/issues/1346

Leave a note about not having tested the scenegraph parser for thread safety