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_test.go

@@ -53,6 +53,10 @@ func TestQueries_GetSystemFeatures(t *testing.T) {
 						context.Background(), aggregate,
 						feature_v2.SystemUserSchemaEventType, false,
 					)),
+					eventFromEventPusher(feature_v2.NewSetEvent(
+						context.Background(), aggregate,
+						feature_v2.SystemEnableRelationalTables, false,
+					)),
 				),
 			),
 			want: &SystemFeatures{
@@ -67,6 +71,10 @@ func TestQueries_GetSystemFeatures(t *testing.T) {
 					Level: feature.LevelSystem,
 					Value: false,
 				},
+				EnableRelationalTables: FeatureSource[bool]{
+					Level: feature.LevelSystem,
+					Value: false,
+				},
 			},
 		},
 		{