BackendPlugins: Point to request Headers in the wrapper (#27599)

In particular, so plugins can see the FromAlert header

pkg/plugins/backendplugin/coreplugin/query_endpoint_adapter.go

@@ -59,6 +59,7 @@ func (a *queryEndpointAdapter) Query(ctx context.Context, ds *models.DataSource,
 			DataSourceInstanceSettings: instanceSettings,
 		},
 		Queries: []backend.DataQuery{},
+		Headers: query.Headers,
 	}
 
 	for _, q := range query.Queries {