lib: add clock class accuracy property

lib: add clock class accuracy property

With the intent of supporting CTF 2, add the accuracy property to clock
classes.

 - Add the bt_clock_class_{g,s}et_accuracy getter and setter, both of
   which are restricted to MIP >= 1.
 - Add the accuracy field to struct bt_clock_class.
 - Make lib-logging print the accuracy of a clock class, if set.

Philippe updated the documentation.

Change-Id: I199ea6d339fcb0142847825c3d12ae526564814d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10711
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12725