Add `rist_sender_npd_get` for querying null-packet-deletion status

Adds a sender function rist_sender_npd_get for getting the current status of null_packet_deletion in client code.

This is added to match flow_id as a sender field that is available to library users. It is worth noting that control of this field is already exposed to the users, through rist_sender_npd_{enable,disable}