bt2: expose seek_ns_from_origin and can_seek_ns_from_origin

bt2: expose seek_ns_from_origin and can_seek_ns_from_origin

This patch makes the seek_ns_from_origin and can_seek_ns_from_origin
operations accessible in Python.  That means:

- A message iterator class implemented in Python can define the
  _user_seek_ns_from_origin and _user_can_seek_ns_from_origin
  methods to implement these operations.
- A message iterator class implemented in Python that has an upstream
  message iterator can call these operations on it using methods on that
  upstream message iterator object.

There is no big surprise in the patch, as everything has mostly been
modeled on seek_beginning/can_seek_beginning.

Change-Id: Ic726b64b4d7c24a59bab03df8a77cf8369bb5e5f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2020
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>