Add support for the new backend system to the RBAC backend plugin.

[davidfestal]

Add support for the new backend system to the RBAC backend plugin.
This implies:

• Creating and providing the BackendFeatureFactory default export
• Thinking about necessary extension points
• Having a way to gather the pluginID of BackendFeatures created with createBackendPlugin(), in order to pass the list of pluginIDs to the RBAC plugin (as an extension ?).

[davidfestal]

related upstream backstage RFC: https://github.com/backstage/backstage/issues/17538