lib: add bt_clock_class_has_same_identity function

lib: add bt_clock_class_has_same_identity function

Add the bt_clock_class_has_same_identity utility function, which tells
if two clock classes have the same identify.

The criteria for whether two clock classes have the same identity are
the same as in CTF 2:

    Two clock classes which satisfy all the following conditions are
    said to have the same identity:

     - Both share the same namespace property value, or both don’t have
       any namespace property.
     - Both have a name property and share the same value.
     - Both have a uid property and share the same value.

Philippe updated the documentation.

Change-Id: I55e4e13274d413a15873099045f14690feab09c4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10749
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12726