Consider adding a managed HorizontalPodAutoscaler

Parent Ticket: https://github.com/apache/solr-operator/issues/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.

Start with the parent ticket and the existing custom HPA path that controls SolrCloud's spec.replicas. Review how the PrometheusExporter would interact with a managed HPA; the work is done when the default HPA properties, optional managed behavior, and compatibility with user-provided HPAs are clearly designed and documented.

Written by the indexing model from the issue text.