Expose client WebSocket connection status

The WebSocket connection status is a critical piece of info when implementing readiness/liveliness probes in kubernetes deployment scenarios. Currently the connection status is kept internally in the `Connection` class and not accessible from outside.

https://github.com/apache/tinkerpop/blob/332177a4b65563bc9ac2ad89e1a3ce78bf290195/gremlin-javascript/src/main/javascript/gremlin-javascript/lib/driver/connection.js#L96

Please let me know if this makes sense and I can send a PR.