feat(rbac): add default role configuration for authenticated users

[JohannesWill]

Hey, I just made a Pull Request!

This PR introduces support for assigning a default role to every authenticated user when using Backstage's Sign-In without Users in the Catalog feature.

Currently, when this option is enabled, the @backstage-community/plugin-rbac-backend cannot evaluate permissions for users not present in the catalog. This limitation is also discussed in issue #2077.

With this change, a configurable default role can be specified and automatically assigned to all authenticated users, enabling consistent permission evaluation even without catalog user entries.

You can assing the role "role:default/my-default-role" to all users

permission:
  rbac:
    defaultRole: role:default/my-default-role

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
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[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-rbac-backend	workspaces/rbac/plugins/rbac-backend	minor	v6.3.0