Allow adding datasources not managed by Juju via config variable

[valexby]

Enhancement Proposal

Hi,

Currently, there are only two feasible ways how to manage Grafana datasources:

1. Add a relation to the Grafana operator charm
2. Manually add datasources via Web GUI or curl

However, the officially recommended go-to option to supply datasources in automative manner is my managing datasources.yaml config file. Would be great to have the ability to manage it via juju config.