Alerting: Introduce proper feature toggles for common state history backend combinations (#65497)

* define 3 feature toggles for rollout phases

* Pass feature toggles along

* Implement first feature toggle

* Try a different strategy with fall-throughs to specific configurations

* Apply toggle overrides once outside of backend composition

* Emit log messages when we coerce backends

* Run code generator for feature toggle files

* Improve wording in flag descs

* Re-run generator

* Use code-generated constants instead of plain strings

* Use converted enum values rather than strings for pre-parsing

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

@@ -87,4 +87,7 @@ export interface FeatureToggles {
   clientTokenRotation?: boolean;
   disableElasticsearchBackendExploreQuery?: boolean;
   prometheusDataplane?: boolean;
+  alertStateHistoryLokiSecondary?: boolean;
+  alertStateHistoryLokiPrimary?: boolean;
+  alertStateHistoryLokiOnly?: boolean;
 }