Fix #0041313

[smeyer-ilias]

This is a proposal to get rid of the outdated php extension xmlrpc
Implementation has been done by @schmitz-ilias

It does not provide full compatibility to the XML-RPC standard but the is a replacement for xmlrpc_en|decode calls used for

• index() index(string $client, bool $bool)
• indexObjects() indexObjects(string $client, array $object_ids)
• ilFO2PDF() ilFO2PDF(string $fo_string)
• searchMail() searchMail(string $client, int $user_id, string $query, int $mail_folder_id)
• highlight() highlight(string $client, array $object_ids, string $query)
• searchUsers() searchUser(string $client, string $query)
• search() search(string $client, string $query, int $page_nr)

https://mantis.ilias.de/view.php?id=41313