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/system_features_model.go

@@ -63,6 +63,7 @@ func (m *SystemFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.SystemEnableBackChannelLogout,
 			feature_v2.SystemLoginVersion,
 			feature_v2.SystemPermissionCheckV2,
+			feature_v2.SystemEnableRelationalTables,
 		).
 		Builder().ResourceOwner(m.ResourceOwner)
 }
@@ -96,6 +97,8 @@ func reduceSystemFeatureSet[T any](features *SystemFeatures, event *feature_v2.S
 		features.LoginV2.set(level, event.Value)
 	case feature.KeyPermissionCheckV2:
 		features.PermissionCheckV2.set(level, event.Value)
+	case feature.KeyEnableRelationalTables:
+		features.EnableRelationalTables.set(level, event.Value)
 	}
 	return nil
 }