Allow users to specify Jetty options

[gerlowskija]

The solrcloud CRD and accompanying helm chart offer ways to set most common bin/solr options, but I recently ran into one of the few exceptions: Jetty options/arguments. As far as I know there's no way to customize the arguments passed to Jetty in a solrcloud, outside of baking certain options into the Docker image itself.

We should find a way to support this usecase. Two initial approaches come to mind:

1. Allow users to override the Docker "command" more broadly. (At which point they can rely on the Solr CLI's -j option)
2. Expose a CRD setting specific to Jetty options that then gets inserted into a Docker "command" template.