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/command/instance_features.go

@@ -23,10 +23,11 @@ type InstanceFeatures struct {
 	LoginV2                        *feature.LoginV2
 	PermissionCheckV2              *bool
 	ConsoleUseV2UserApi            *bool
+	EnableRelationalTables         *bool
 }
 
 func (m *InstanceFeatures) isEmpty() bool {
-	return m.LoginDefaultOrg == nil &&
+	return m == nil || (m.LoginDefaultOrg == nil &&
 		m.UserSchema == nil &&
 		m.TokenExchange == nil &&
 		// nil check to allow unset improvements
@@ -35,7 +36,9 @@ func (m *InstanceFeatures) isEmpty() bool {
 		m.OIDCSingleV1SessionTermination == nil &&
 		m.EnableBackChannelLogout == nil &&
 		m.LoginV2 == nil &&
-		m.PermissionCheckV2 == nil && m.ConsoleUseV2UserApi == nil
+		m.PermissionCheckV2 == nil &&
+		m.ConsoleUseV2UserApi == nil &&
+		m.EnableRelationalTables == nil)
 }
 
 func (c *Commands) SetInstanceFeatures(ctx context.Context, f *InstanceFeatures) (*domain.ObjectDetails, error) {