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 <27845747+adlerhurst@users.noreply.github.com>

internal/query/projection/instance_features.go

@@ -104,6 +104,10 @@ func (*instanceFeatureProjection) Reducers() []handler.AggregateReducer {
 				Event:  instance.InstanceRemovedEventType,
 				Reduce: reduceInstanceRemovedHelper(InstanceDomainInstanceIDCol),
 			},
+			{
+				Event:  feature_v2.InstanceEnableRelationalTables,
+				Reduce: reduceInstanceSetFeature[bool],
+			},
 		},
 	}}
 }