Start of dashboard query API (#49547)

This PR adds endpoints for public dashboards to retrieve data from the backend (trusted) query engine. It works by executing queries defined on the backend without any user input and does not support template variables.

* Public dashboard query API
* Create new API on service for building metric request
* Flesh out testing, implement BuildPublicDashboardMetricRequest
* Test for errors and missing panels
* Refactor tests, add supporting code for multiple datasources
* Handle queries from multiple datasources
* Explicitly pass no user for querying public dashboard

Co-authored-by: Jeff Levin <jeff@levinology.com>

pkg/api/api.go

@@ -613,6 +613,7 @@ func (hs *HTTPServer) registerRoutes() {
 	// Public API
 	if hs.Features.IsEnabled(featuremgmt.FlagPublicDashboards) {
 		r.Get("/api/public/dashboards/:uid", routing.Wrap(hs.GetPublicDashboard))
+		r.Post("/api/public/dashboards/:uid/panels/:panelId/query", routing.Wrap(hs.QueryPublicDashboard))
 	}
 
 	// Frontend logs