lib: add clock class namespace and UID properties

lib: add clock class namespace and UID properties

Add the namespace and UID properties to clock class objects.  This is
needed to support CTF 2.

Philippe updated the documentation.

Change-Id: I9b6ee8f78581ca8ebfa8b1ceca0d1fe50178c1b3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10677
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12720