Dashboard to allow storing references

[streamich]

TLDR; integrate PersistableStateService into dashboard.

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Dashboard, e.g. SavedDashboard, should expose a way provided references to other saved objects in its update/create methods. Currently, only references to dashboard visualizations are maintained in a hard-coded extractReferences function.

But dashboard could contain references to also other saved objects besides visualizations. Currently, embeddables manage the state of each panel of the dashboard and that state could reference other saved objects, in which case there should be a way for embeddable to notify the dashboard, e.g. SavedDashboard, about the references it contains.

• Should be possible to provide SO references on dashboard update/save.
• Should be done in a way such that it would work with embeddables.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ThomThomson]

Closing this as completed because Dashboard is integrated with the persistable state service.