6187-DS-RECORD-functions-for-API -> Update dns.inc.php and other related files

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' -> functions added : dns_ds_get, dns_ds_add, dns_ds_update, dns_ds_delete

Other files added : dns_ds_get.php, dns_ds_get.html, dns_ds_add.php, dns_ds_add.html, dns_ds_update.php, dns_ds_update.html, dns_ds_delete.php, dns_ds_delete.html -> These files are related to new functions added

Closes #6187 (closed)