feat: add system field to data source instances

[darrenjaneczek]

What this PR does / why we need it:

This PR proposes to add a System field to DataSourceInstanceSettings, which will be used by Grafana to indicate that the datasource instance should be hidden from view.

This PR is a dependency of the functionality here:

• feat: add system field to data source instances and types grafana#97315

Example uses of "system" data sources

1:n proxies

Sometimes plugin developers need the ability to proxy to multiple services of the same type. By creating multiple data sources, an app plugin can achieve this. The app plugin would need to take responsibility for creating and configuring their system data sources.

General persistence

An additional functionality that hidden datasource instances could provide, is in persist bits of JSON data. In certain environments (like Grafana cloud), the provisioning mechanism will completely rewrite the jsonData of provisioned plugins -- having a data source instantiated allows an app plugin to store some state that will not be automatically deleted in instance refresh.

The desire to make them invisible

Currently data sources can be used to provide the above uses. However, it might not be suitable to view these along with other data sources in typical Grafana views. If there is no need to access these "data sources" in a dashboard, or in the Explore view, adding this "system" flag, a developer can hide these data sources.

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