lib: add trace namespace property

lib: add trace namespace property

Add a new namespace property for trace objects.  This is needed to
support CTF 2.

Philippe updated the documentation.

Change-Id: I9c382879a3181aab2859a4c41cfbb21c4f6928aa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10695
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12721