Support for custom Neo4J authentication

[michaelnchin]

Issue #, if available: #314

Description of changes:

• Added new neo4j section to %%graph_notebook_config for Neo4J authentication options. In this section, users can now specify a username, password, and whether auth is enabled on the Neo4J endpoint.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.