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/api/grpc/feature/v2/feature.go

@@ -11,7 +11,11 @@ import (
 )
 
 func (s *Server) SetSystemFeatures(ctx context.Context, req *feature.SetSystemFeaturesRequest) (_ *feature.SetSystemFeaturesResponse, err error) {
-	details, err := s.command.SetSystemFeatures(ctx, systemFeaturesToCommand(req))
+	features, err := systemFeaturesToCommand(req)
+	if err != nil {
+		return nil, err
+	}
+	details, err := s.command.SetSystemFeatures(ctx, features)
 	if err != nil {
 		return nil, err
 	}
@@ -39,7 +43,11 @@ func (s *Server) GetSystemFeatures(ctx context.Context, req *feature.GetSystemFe
 }
 
 func (s *Server) SetInstanceFeatures(ctx context.Context, req *feature.SetInstanceFeaturesRequest) (_ *feature.SetInstanceFeaturesResponse, err error) {
-	details, err := s.command.SetInstanceFeatures(ctx, instanceFeaturesToCommand(req))
+	features, err := instanceFeaturesToCommand(req)
+	if err != nil {
+		return nil, err
+	}
+	details, err := s.command.SetInstanceFeatures(ctx, features)
 	if err != nil {
 		return nil, err
 	}