Enable assigning non-custom and custom stakeholders in registration

Administrators can now decide whether certain roles, such as the model owner, are optional or required during model registration.

Manage model stakeholder types

This change ensures that critical responsibilities are always assigned. The model creation form has also been redesigned to make stakeholder selection more straightforward.

Additionally, organization settings now offer a simpler way to manage role requirements. Administrators can configure these directly through the interface, making it easier to keep the system aligned with governance policies and organizational needs.