Refactor some variable names in the grafana_source library

[lucabello]

Enhancement Proposal

The grafana_source library, and specifically GrafanaSourceProvider, provides some constructor arguments which should be refactored, namely:

• extra_fields
• secure_extra_fields

When they are translated in datasource configuration from Grafana, in _generate_datasource_config() they are converted in jsonData and secureJsonData respectively.

This requires change in the Provider, possibly in the Requirer as well (to read them from relation data?) and in the charm.

Thus, the name of the constructor arguments (and probably the variables) should be refacored to match the field names in the datasource configuration, meaning:

• extra_fields => json_data
• secure_extra_fields => secure_json_data