dashboard: Investigate using grafonnet for building dashboards

[david-martin]

• https://github.com/grafana/grafonnet
• Identify pros & cons of using it vs. the current approach (building in UI then exporting json)

One potential pro I'd like to bottom out is if parts of a dashboard can changed dynamically based on input variables when building them.
For example, including drill down links from individual HTTPRoutes to a gateway provider specific dashboard, like the Istio Workload dashboard pre-filtered to that route.

[david-martin]

closed by #35