[Dashboards] Export source (serializable state) of a dashboard

[nickpeihl]

Fixes #192913
WIP
Blocked by #217109

Summary

Adds a flyout that can export the serializable state (JSON source) of a dashboard.

Since the flyout uses the dashboardApi.getSerializedState method, it is not necessary to save a dashboard before exporting the source. The state can be copied and pasted into the request body of the Dashboard HTTP API, e.g.

POST kbn:/api/dashboards/dashboard
{
  "attributes": ...,
  "references": ...
}

TODO

• Wire into the upcoming export menu
• Remove the current changes to the dashboard top nav as this is just a demo

This PR also adds a re-useable @kbn/content-management-content-source package that other plugins may use to show their content source.

[elasticmachine]

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[nickpeihl]

Superceded by #221780