Add STUB endpoints for settings RBAC (kubeflow#982)

- Introduced new API endpoints for managing certificates and role bindings.
- Updated the model registry settings to include optional credentials.
- Added new response types and payload structures for certificates and role bindings.
- Enhanced existing handlers to support the new functionality.

Signed-off-by: lucferbux <lferrnan@redhat.com>

Author: lucferbux