CLI tool for testing autoscaling suggestions against a live cluster

Solr already provides /autoscaling/diagnostics and /autoscaling/suggestions endpoints. In some situations it would be very helpful to be able to run "what if" scenarios using data about nodes and replicas taken from a production cluster but with a different autoscaling policy than the one that is deployed, without also worrying that the calculations would negatively impact a production cluster's Overseer leader.

All necessary classes (including the Policy engine) are self-contained in the SolrJ component, so it's just a matter of packaging and writing a CLI tool + a wrapper script.