Supporting Java operators

[tianyin]

@MarkintoshZ is asking whether he should use Acto to test a new operator. I think it's a great idea and can really help him develop a holistic understanding of Acto.

To make it more challenging, my suggestion is to support Java operators -- we always claim that Acto is generic to any languages but all what we have done has been for Go operators.

@Spedoske worked on a Java operator (strimzi-kafka-cluster-operator) but didn't finish it. See the following issue: https://github.com/xlab-uiuc/acto/issues/230

@MarkintoshZ Can you finish it?

Note that it will be quite some challenges.

Some of the half baked code can be found in, https://github.com/xlab-uiuc/acto/pull/229

and the latest progress is in https://github.com/xlab-uiuc/acto/issues/230