roster
Retrieves the user roster.
In XMPP, one's contact list is called a roster, which consists of any number of specific roster items, each roster item being identified by a unique JID (usually of the form contact@domain). For more information, refer to https://xmpp.org/rfcs/rfc6121.html#intro-requirements
Return
The user roster as an ArrayItemList of IRainbowContact.