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/feature/feature.go

@@ -22,6 +22,7 @@ const (
 	KeyLoginV2                        Key = 13
 	KeyPermissionCheckV2              Key = 14
 	KeyConsoleUseV2UserApi            Key = 15
+	KeyEnableRelationalTables         Key = 16
 )
 
 //go:generate enumer -type Level -transform snake -trimprefix Level
@@ -49,6 +50,7 @@ type Features struct {
 	LoginV2                        LoginV2                   `json:"login_v2,omitempty"`
 	PermissionCheckV2              bool                      `json:"permission_check_v2,omitempty"`
 	ConsoleUseV2UserApi            bool                      `json:"console_use_v2_user_api,omitempty"`
+	EnableRelationalTables         bool                      `json:"enable_relational_tables,omitempty"`
 }
 
 /* Note: do not generate the stringer or enumer for this type, is it breaks existing events */