ctf plugin: add bt_ctf_notif_iter_get_packet_header_context_fields()

The new bt_ctf_notif_iter_get_packet_header_context_fields() function
returns the packet header and context fields using the usual medium
operation mechanism without ever creating packet of event objects,
therefore not requiring a stream object to exist.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>