loki: generate dataplane-compliant logs dataframes (behind feature flag) (#69909)

* add feature flag

* prepare tests

* enable dataplane-format in loki-backend

packages/grafana-data/src/types/featureToggles.gen.ts

@@ -78,6 +78,7 @@ export interface FeatureToggles {
   clientTokenRotation?: boolean;
   prometheusDataplane?: boolean;
   lokiMetricDataplane?: boolean;
+  lokiLogsDataplane?: boolean;
   dataplaneFrontendFallback?: boolean;
   disableSSEDataplane?: boolean;
   alertStateHistoryLokiSecondary?: boolean;