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_model_test.go

@@ -0,0 +1,144 @@
+package command
+
+import (
+	"testing"
+
+	"github.com/muhlemmer/gu"
+	"github.com/stretchr/testify/assert"
+
+	"github.com/zitadel/zitadel/internal/feature"
+)
+
+func Test_reduceInstanceFeature(t *testing.T) {
+	type args struct {
+		features *InstanceFeatures
+		key      feature.Key
+		value    any
+	}
+	tt := []struct {
+		name     string
+		args     args
+		expected *InstanceFeatures
+	}{
+		{
+			name: "key unspecified",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyUnspecified,
+				value:    true,
+			},
+			expected: &InstanceFeatures{},
+		},
+		{
+			name: "login default org",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyLoginDefaultOrg,
+				value:    true,
+			},
+			expected: &InstanceFeatures{LoginDefaultOrg: gu.Ptr(true)},
+		},
+		{
+			name: "token exchange",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyTokenExchange,
+				value:    true,
+			},
+			expected: &InstanceFeatures{TokenExchange: gu.Ptr(true)},
+		},
+		{
+			name: "user schema",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyUserSchema,
+				value:    true,
+			},
+			expected: &InstanceFeatures{UserSchema: gu.Ptr(true)},
+		},
+		{
+			name: "improved performance",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyImprovedPerformance,
+				value:    []feature.ImprovedPerformanceType{feature.ImprovedPerformanceTypeProject},
+			},
+			expected: &InstanceFeatures{ImprovedPerformance: []feature.ImprovedPerformanceType{feature.ImprovedPerformanceTypeProject}},
+		},
+		{
+			name: "debug OIDC parent error",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyDebugOIDCParentError,
+				value:    true,
+			},
+			expected: &InstanceFeatures{DebugOIDCParentError: gu.Ptr(true)},
+		},
+		{
+			name: "OIDC single v1 session termination",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyOIDCSingleV1SessionTermination,
+				value:    true,
+			},
+			expected: &InstanceFeatures{OIDCSingleV1SessionTermination: gu.Ptr(true)},
+		},
+		{
+			name: "enable back channel logout",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyEnableBackChannelLogout,
+				value:    true,
+			},
+			expected: &InstanceFeatures{EnableBackChannelLogout: gu.Ptr(true)},
+		},
+		{
+			name: "login v2",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyLoginV2,
+				value:    &feature.LoginV2{},
+			},
+			expected: &InstanceFeatures{LoginV2: &feature.LoginV2{}},
+		},
+		{
+			name: "permission check v2",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyPermissionCheckV2,
+				value:    true,
+			},
+			expected: &InstanceFeatures{PermissionCheckV2: gu.Ptr(true)},
+		},
+		{
+			name: "console use v2 user api",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyConsoleUseV2UserApi,
+				value:    true,
+			},
+			expected: &InstanceFeatures{ConsoleUseV2UserApi: gu.Ptr(true)},
+		},
+		{
+			name: "enable relational tables",
+			args: args{
+				features: &InstanceFeatures{},
+				key:      feature.KeyEnableRelationalTables,
+				value:    true,
+			},
+			expected: &InstanceFeatures{EnableRelationalTables: gu.Ptr(true)},
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			// Test
+			reduceInstanceFeature(tc.args.features, tc.args.key, tc.args.value)
+
+			// Verify
+			assert.Equal(t, tc.expected, tc.args.features)
+		})
+	}
+}