Return certificate name within get_client_config api call

We should also return certificate name so we could use it on frontend when offering download of such file.

Just add name to json schema reply:

https://gitlab.nic.cz/turris/foris-controller/foris-controller-openvpn-module/-/blob/master/foris_controller_modules/openvpn/schema/openvpn.json#L426

We could reuse or get inspiration by this object: https://gitlab.nic.cz/turris/foris-controller/foris-controller-openvpn-module/-/blob/master/foris_controller_modules/openvpn/schema/openvpn.json#L84

Dependency for: turris/reforis/reforis-openvpn#17 (closed)