lib: add namespace property to stream classes

lib: add namespace property to stream classes

Add the namespace property to stream classes, with the intent to support
namespace properties on data stream classes in CTF2‑SPEC‑2.0 [1].

Copy everything from the existing name property, and rename it to
"namespace."

Both new public functions have a `MIP >= 1` pre-condition check.

Philippe updated the documentation.

[1] https://diamon.org/ctf/CTF2-SPEC-2.0.html

Change-Id: Ibdfc9c924fc6347f5020c5ba06f14a7737cbf76b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7380
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12688