Gremlin-JavaScript: Pass custom headers to the websocket connection

In some cases, it is required to pass extra header information to the Gremlin server to successfully make a connection. This ticket is to add the feature to enable users to optionally pass headers via the JavaScript client when connecting to a Gremlin Server.

This was originally noted when connecting to Amazon Neptune with the SIGv4 signing required when using IAM-based authentication, but is generally applicable for use of the JavaScript client with Gremlin Servers that require additional header information. 

The corresponding PR is below.

Pull Request