Add read-only permission for /api/v3/rbac/permissions/users

**Is your feature request related to a problem? Please describe.**
We have an automated, regular check that compares our Terraform IaC configuration against the actual state of the Authentik setup. The user for this has exclusively read-only permissions to the API. As far as I can tell, accessing `/api/v3/rbac/permissions/users` is not possible without super user privileges.

**Describe the solution you'd like**
Add a permission that allows `GET` on `/api/v3/rbac/permissions/users` without requiring super-user privileges or any other write permissions.

**Describe alternatives you've considered**
Elevating privileges is out of the question.