Consider adding a managed HorizontalPodAutoscaler

[HoustonPutman]

Parent Ticket: #536

Currently users can create their own HPA, and have it be used to control a SolrCloud's spec.replicas parameter.

However if we want to support Autoscaling out-of-the-box, and make it as easy as possible for users to try, then it would be a good idea to add an optional managed HPA. This way users could use their custom HPA or use the built in.

There are many questions to be answered around the default properties of this HPA, and given that it would need to interact with the PrometheusExporter, things start to get a little iffy.

[HoustonPutman]

This would basically just be templating the HPA for users. There's really nothing much the Operator can add here.

Instead we might consider just adding an option to the Solr helm chart. Much lighter weight.

[HoustonPutman]

Removing the milestone, as this isn't a priority. If needed, I think we should go with the helm chart option in the future.