Apply for rules are defined through `apply_for` property
Only Array custom variables will be eligible in apply_for dropdown.
API Example:
```bash
./director-curl POST director/service?name=my_service '{"apply_for": "for (checked_tcp_port in host.vars.checked_tcp_ports)" }'
```
Will render as:
```
apply Service "my_service" for (checked_tcp_port in host.vars.checked_tcp_ports) {
...
}
```
Feature: https://dev.icinga.org/issues/11976
Depends-On: https://github.com/Icinga/icingaweb2-module-director/pull/20
refs #11976
Thomas Gelf
Corentin Ardeois