feat: specify login UI version on instance and apps (#9071)

# Which Problems Are Solved

To be able to migrate or test the new login UI, admins might want to
(temporarily) switch individual apps.
At a later point admin might want to make sure all applications use the
new login UI.

# How the Problems Are Solved

- Added a feature flag `` on instance level to require all apps to use
the new login and provide an optional base url.
- if the flag is enabled, all (OIDC) applications will automatically use
the v2 login.
  - if disabled, applications can decide based on their configuration
- Added an option on OIDC apps to use the new login UI and an optional
base url.
- Removed the requirement to use `x-zitadel-login-client` to be
redirected to the login V2 and retrieve created authrequest and link
them to SSO sessions.
- Added a new "IAM_LOGIN_CLIENT" role to allow management of users,
sessions, grants and more without `x-zitadel-login-client`.

# Additional Changes

None

# Additional Context

closes https://github.com/zitadel/zitadel/issues/8702

internal/command/system_features_model.go

@@ -34,6 +34,12 @@ func (m *SystemFeaturesWriteModel) Reduce() error {
 				return err
 			}
 			reduceSystemFeature(&m.SystemFeatures, key, e.Value)
+		case *feature_v2.SetEvent[*feature.LoginV2]:
+			_, key, err := e.FeatureInfo()
+			if err != nil {
+				return err
+			}
+			reduceSystemFeature(&m.SystemFeatures, key, e.Value)
 		case *feature_v2.SetEvent[[]feature.ImprovedPerformanceType]:
 			_, key, err := e.FeatureInfo()
 			if err != nil {
@@ -63,6 +69,7 @@ func (m *SystemFeaturesWriteModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.SystemOIDCSingleV1SessionTerminationEventType,
 			feature_v2.SystemDisableUserTokenEvent,
 			feature_v2.SystemEnableBackChannelLogout,
+			feature_v2.SystemLoginVersion,
 		).
 		Builder().ResourceOwner(m.ResourceOwner)
 }
@@ -104,6 +111,8 @@ func reduceSystemFeature(features *SystemFeatures, key feature.Key, value any) {
 	case feature.KeyEnableBackChannelLogout:
 		v := value.(bool)
 		features.EnableBackChannelLogout = &v
+	case feature.KeyLoginV2:
+		features.LoginV2 = value.(*feature.LoginV2)
 	}
 }
 
@@ -120,6 +129,7 @@ func (wm *SystemFeaturesWriteModel) setCommands(ctx context.Context, f *SystemFe
 	cmds = appendFeatureUpdate(ctx, cmds, aggregate, wm.OIDCSingleV1SessionTermination, f.OIDCSingleV1SessionTermination, feature_v2.SystemOIDCSingleV1SessionTerminationEventType)
 	cmds = appendFeatureUpdate(ctx, cmds, aggregate, wm.DisableUserTokenEvent, f.DisableUserTokenEvent, feature_v2.SystemDisableUserTokenEvent)
 	cmds = appendFeatureUpdate(ctx, cmds, aggregate, wm.EnableBackChannelLogout, f.EnableBackChannelLogout, feature_v2.SystemEnableBackChannelLogout)
+	cmds = appendFeatureUpdate(ctx, cmds, aggregate, wm.LoginV2, f.LoginV2, feature_v2.SystemLoginVersion)
 	return cmds
 }