radiusproxy: add permission for reading radius proxy servers
A non-admin user which has the "User Administrator" role cannot
add a user with ipa user-add --radius=<proxy> because the
call needs to read the radius proxy server entries.
The fix adds a System permission for reading radius proxy server
entries (all attributes except the ipatokenradiussecret). This
permission is added to the already existing privileges "User
Administrators" and "Stage User Administrators", so that the role
"User Administrator" can call ipa [stage]user-add|mod --radius=<proxy>
Fixes: https://pagure.io/freeipa/issue/7570
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>