[FEATURE REQUEST] add SecureJsonData argument in grafana4_datasource state and module

[keenobi]

Is your feature request related to a problem? Please describe.

We want to create a datasource with an HTTP header. But in Grafana's API call the header value must be provided in the secureJsonData field. https://grafana.com/docs/grafana/latest/developers/http_api/data_source/#create-a-data-source

Describe the solution you'd like

We would like to have a secure_json_data attribute to be able to set the desired values.

Additional context

We will propose the PR soon.

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