This method will retrieve this entity's roster from the server.
async xmpp.retrieveRoster(): Promise<void>
After the class connects, it will automatically send a request to the server to retrieve the roster. However, should the user or application wish to update the entire roster, this method may be used to do so.
After a successful call to the RetrieveRoster method, the server will respond with this entity's roster. The class will parse the roster and fire the Sync event once per item. This event may also fire for each entity added to or removed from the roster.