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/query/system_features_model.go

@@ -32,6 +32,11 @@ func (m *SystemFeaturesReadModel) Reduce() error {
 			if err != nil {
 				return err
 			}
+		case *feature_v2.SetEvent[*feature.LoginV2]:
+			err := reduceSystemFeatureSet(m.system, e)
+			if err != nil {
+				return err
+			}
 		case *feature_v2.SetEvent[[]feature.ImprovedPerformanceType]:
 			err := reduceSystemFeatureSet(m.system, e)
 			if err != nil {
@@ -60,6 +65,7 @@ func (m *SystemFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.SystemOIDCSingleV1SessionTerminationEventType,
 			feature_v2.SystemDisableUserTokenEvent,
 			feature_v2.SystemEnableBackChannelLogout,
+			feature_v2.SystemLoginVersion,
 		).
 		Builder().ResourceOwner(m.ResourceOwner)
 }
@@ -97,6 +103,8 @@ func reduceSystemFeatureSet[T any](features *SystemFeatures, event *feature_v2.S
 		features.DisableUserTokenEvent.set(level, event.Value)
 	case feature.KeyEnableBackChannelLogout:
 		features.EnableBackChannelLogout.set(level, event.Value)
+	case feature.KeyLoginV2:
+		features.LoginV2.set(level, event.Value)
 	}
 	return nil
 }