Zanzana: database migrations (#89390)

* Zanana: Use grafana migrations to run openFGA migration files and initilize store.

* Add feature toggle

* Zanzana: return noop client if feature toggle is disabled

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

@@ -197,4 +197,5 @@ export interface FeatureToggles {
   openSearchBackendFlowEnabled?: boolean;
   ssoSettingsLDAP?: boolean;
   databaseReadReplica?: boolean;
+  zanzana?: boolean;
 }