API: Add by UID routes for data sources (#29884)

- also add Get by UID+OrgID to datasource cache
- Refactor backend commands for Delete and Get to be unified

pkg/services/alerting/alerting_usage_test.go

@@ -38,7 +38,7 @@ func TestAlertingUsageStats(t *testing.T) {
 		return nil
 	})
 
-	ae.Bus.AddHandler(func(query *models.GetDataSourceByIdQuery) error {
+	ae.Bus.AddHandler(func(query *models.GetDataSourceQuery) error {
 		ds := map[int64]*models.DataSource{
 			1: {Type: "influxdb"},
 			2: {Type: "graphite"},