Update dns.inc.php

Following below discussion (link) I propose to add DS RECORD functions for API
https://www.howtoforge.com/community/threads/dnssec-cascade-inside-ispconfig.86988/#post-423182
code added in 'interface/lib/classes/remote.d/dns.inc.php'
// ----------------------------------------------------------------------------------------------------------------
//* Get record details
public function dns_ds_get($session_id, $primary_id) {
return $this->dns_rr_get($session_id, $primary_id, 'DS');
}
//* Add a record
public function dns_ds_add($session_id, $client_id, $params, $update_serial=false) {
return $this->dns_rr_add($session_id, $client_id, $params, $update_serial, 'DS');
}
//* Update a record
public function dns_ds_update($session_id, $client_id, $primary_id, $params, $update_serial=false) {
return $this->dns_rr_update($session_id, $client_id, $primary_id, $params, $update_serial, 'DS');
}
//* Delete a record
public function dns_ds_delete($session_id, $primary_id, $update_serial=false) {
return $this->dns_rr_delete($session_id, $primary_id, $update_serial, 'DS');
}