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/instance_features_model.go

@@ -42,6 +42,8 @@ func (m *InstanceFeaturesReadModel) Reduce() (err error) {
 			)
 		case *feature_v2.SetEvent[bool]:
 			err = reduceInstanceFeatureSet(m.instance, e)
+		case *feature_v2.SetEvent[*feature.LoginV2]:
+			err = reduceInstanceFeatureSet(m.instance, e)
 		case *feature_v2.SetEvent[[]feature.ImprovedPerformanceType]:
 			err = reduceInstanceFeatureSet(m.instance, e)
 		}
@@ -72,6 +74,7 @@ func (m *InstanceFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.InstanceOIDCSingleV1SessionTerminationEventType,
 			feature_v2.InstanceDisableUserTokenEvent,
 			feature_v2.InstanceEnableBackChannelLogout,
+			feature_v2.InstanceLoginVersion,
 		).
 		Builder().ResourceOwner(m.ResourceOwner)
 }
@@ -98,6 +101,7 @@ func (m *InstanceFeaturesReadModel) populateFromSystem() bool {
 	m.instance.OIDCSingleV1SessionTermination = m.system.OIDCSingleV1SessionTermination
 	m.instance.DisableUserTokenEvent = m.system.DisableUserTokenEvent
 	m.instance.EnableBackChannelLogout = m.system.EnableBackChannelLogout
+	m.instance.LoginV2 = m.system.LoginV2
 	return true
 }
 
@@ -133,6 +137,8 @@ func reduceInstanceFeatureSet[T any](features *InstanceFeatures, event *feature_
 		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
 }