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/instance_features.go

@@ -19,6 +19,7 @@ type InstanceFeatures struct {
 	LoginV2                        FeatureSource[*feature.LoginV2]
 	PermissionCheckV2              FeatureSource[bool]
 	ConsoleUseV2UserApi            FeatureSource[bool]
+	EnableRelationalTables         FeatureSource[bool]
 }
 
 func (q *Queries) GetInstanceFeatures(ctx context.Context, cascade bool) (_ *InstanceFeatures, err error) {