add kafka producer cli command

[miazn]

Wanted an easy way to call my new kafka producer in isolation from snarkOS, so decided to create a new cli command.

This works by adding a dir to the existing cli folder and allowing the message to be inserted in cli arguments. Note that you need to run docker-compose up for this to work.

Also you need to declare the topic names ahead of time (harder to declare using the rust script itself). The way we might do this on the node is to write an envsubst script for the docker-compose file itself on node startup.

Also note that these kafka brokers are not encrypted, should add before pushing to node