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/command/instance.go

@@ -141,6 +141,7 @@ type InstanceSetupFeatures struct {
 	LoginV2                        *InstanceSetupFeatureLoginV2
 	PermissionCheckV2              *bool
 	ConsoleUseV2UserApi            *bool
+	EnableRelationalTables         *bool
 }
 
 type InstanceSetupFeatureLoginV2 struct {
@@ -174,6 +175,7 @@ func (f *InstanceSetupFeatures) ToInstanceFeatures() (_ *InstanceFeatures, err e
 		LoginV2:                        loginV2,
 		PermissionCheckV2:              f.PermissionCheckV2,
 		ConsoleUseV2UserApi:            f.ConsoleUseV2UserApi,
+		EnableRelationalTables:         f.EnableRelationalTables,
 	}, nil
 }