Add viewers_can_edit parameter to Grafana-K8s charm configurations

[henryoudaimy]

Enhancement Proposal

The purpose of this enhancement is to add the config param "viewers_can_edit" which can be set to true/false, similarly in the grafana config file /usr/local/etc/grafana/grafana.ini according to the official Grafana documentation
This is currently not supported in the charm, and by enabling/disabling this parameter, this should allow/forbid users with "view" role to access the Grafana "Explore" feature