Support External Environment For Java SDK Harness

As mentioned in this thread  the Java SDK harness only supports environments docker and embedded for the Java SDK harness.

It would be nice if the Java SDK also supported external environments similar to what the Python SDK harness does.

The use case would be running in Kubernetes. On Kubernetes I think its better to run the SDK harness containers as side cars of the taskmanager pods rather than relying on docker in docker. Docker in docker isn't great from a security standpoint because it requires privileged containers. It also makes it harder to debug and operate on Kubernetes.

This was also mentioned in this thread on the developer mailing list; thread