Feature Overview (aka. Goal Summary)

Support defining defaultRole  via RBAC `app-config.yaml`.

This feature partially addresses these requirements:

1. When using Backstage's Sign-In without Users in the Catalog feature, the @backstage-community/plugin-rbac-backend cannot evaluate permissions for users not present in the catalog
2. Avoid 'all-employee' groups being required mentioned in wildcard support request: https://issues.redhat.com/browse/RHIDP-6493 

This request comes from community (https://github.com/backstage/community-plugins/issues/5358) and contributions have already been made for it:

1. default role: https://github.com/backstage/community-plugins/pull/4350
2. default permissions: https://github.com/backstage/community-plugins/pull/3908 

Goals (aka. expected user outcomes)

The goal is to support defining defaultRole via configuration file for all authenticated users.

Requirements (aka. Acceptance Criteria):

• As an admin, I want to be able to define a default role directly within the `app-config.yaml` file, so that I don't have to manually create all users group and assign all users to it

Customer Considerations (Optional)

The RBAC plugin currently requires explicit role assignment for every user or group to receive permissions. For large organizations, this creates a significant administrative burden when you want to provide baseline permissions to all authenticated users.

Currently, to provide baseline permissions to all users in a Backstage instance, you must:

1. Create a group containing all users, or
2. Use the admin users configuration (which grants unlimited access), or
3. Manually assign every individual user to a role

For organizations with large user bases, none of these approaches are scalable or maintainable:

• Group-based approach: Requires maintaining group membership for every user
• Admin Users: Provides unlimited access, which is too permissive for baseline permissions
• Individual assignment: Not feasible for large user counts

Documentation Considerations

• As a platform engineer, I want to be able to find information on how to configure default role and permissions via configuration file.