Add dashboard for oncall metrics to provisioning plugin settings (#1973)

Add dashboard for oncall metrics to the plugin settings by following
[this
instruction](https://community.grafana.com/t/how-to-include-dashboards-in-your-data-source-plugin/62749).
Dashboards files should be in `.json` format and stored in the plugin's
directory in `/src/dashboards/`.
As the result the dashboard can be imported from the `Dashboards` tab at
the plugin settings page:
<img width="1504" alt="Screenshot 2023-06-21 at 11 28 31"
src="https://github.com/grafana/oncall/assets/22730923/8c2bfe4a-f3c8-480a-b18c-7ba21a9b8e30">

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Co-authored-by: Joey Orlando <joey.orlando@grafana.com>