[Dashboard][Service] Expose the dashboard service from the server plugin contract

[mohamedhamed-ahmed]

📝 Description

The FindDashboardsService is used to query dashboards from SavedObjects but at the moment its only being exposed from public plugin start contract.
To allow others to use the service on the server side without directly querying the dashboard SavedObject we need this service to also be exposed from the server plugin contract.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[ThomThomson]

Closing this issue and adding it to our icebox. It's unlikely that we'll be able to get to this in the near term, but we'll continue to track it in our icebox.