Allow user to provide their own Grafana instance

It would be useful to have the option to integrate with an existing Grafana pod. The user would have to ensure that the datasource and Cryostat Dashboard are installed. Perhaps this can be improved by making our customizations into a Grafana plugin. Since this would involve datasource traffic leaving the pod, we'd have to set up TLS for the datasource container as well.

No files or tests are named. Start by tracing how the operator installs and configures Grafana, its datasource, and the Cryostat Dashboard, then compare the external-pod, plugin, and TLS requirements. Done means a documented, tested design and implementation for using a user-provided Grafana instance without losing required datasource security.

Written by the indexing model from the issue text.