feat: Feature flag for relational tables (#10599)

# Which Problems Are Solved

This PR introduces a new feature flag `EnableRelationalTables` that will
be used in following implementations to decide whether Zitadel should
use the relational model or the event sourcing one.

# TODO

  - [x] Implement flag at system level
- [x] Display the flag on console:
https://github.com/zitadel/zitadel/pull/10615

# How the Problems Are Solved

  - Implement loading the flag from config
- Add persistence of the flag through gRPC endpoint
(SetInstanceFeatures)
- Implement reading of the flag through gRPC endpoint
(GetInstanceFeatures)

# Additional Changes

Some minor refactoring to remove un-needed generics annotations

# Additional Context

- Closes #10574

---------

Co-authored-by: Silvan <[email protected]>

internal/query/instance_features_model.go

@@ -72,11 +72,13 @@ func (m *InstanceFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.InstanceLoginVersion,
 			feature_v2.InstancePermissionCheckV2,
 			feature_v2.InstanceConsoleUseV2UserApi,
+			feature_v2.InstanceEnableRelationalTables,
 		).
 		Builder().ResourceOwner(m.ResourceOwner)
 }
 
 func (m *InstanceFeaturesReadModel) reduceReset() {
+	m.instance.EnableRelationalTables = FeatureSource[bool]{}
 	if m.populateFromSystem() {
 		return
 	}
@@ -126,6 +128,8 @@ func reduceInstanceFeatureSet[T any](features *InstanceFeatures, event *feature_
 		features.PermissionCheckV2.set(level, event.Value)
 	case feature.KeyConsoleUseV2UserApi:
 		features.ConsoleUseV2UserApi.set(level, event.Value)
+	case feature.KeyEnableRelationalTables:
+		features.EnableRelationalTables.set(level, event.Value)
 	}
 	return nil
 }