add optional secureJsonData to grafana datasources

[lucabello]

Issue

Closes #295.

Some datasources, like Mimir, need custom HTTP Headers to be specified in the datasource configuration.

As seen here, header names can be specified under jsonData, which we currently build from the extra_data parameter in the GrafanaSourceProvider constructor.

However, the value for a header must be specified under secureJsonData, which we currently don't allow to specify in the grafana_source library.

Solution

This PR adds an optional secure_extra_data constructor argument, right next to extra_data, so that we can (in the same fashion as jsonData) specify secureJsonData for a datasource.

[lucabello]

Headers are set successfully now:

And the Mimir datasource is happy as a consequence:

[PietroPasotti]

looks grafana to me