Get GEL running as a "backend datasource"

[kylebrandt]

We currently have backend plugins that Grafana will start the process for once they are built. For example, see the servicenow-datasource baseline branch with datasource.go: https://github.com/grafana/grafana-servicenow-datasource/tree/20190723_baseline

Even if this ends up not being exactly a datasource plugin, and can't actually do anything with any calls made from the front end - but this positions the plumbing so we are closer I think.