lib: add bt_trace_{get,set}_uid

lib: add bt_trace_{get,set}_uid

In MIP 1, the trace UID replaces the trace UUID.  A trace UID is an
arbitrary string.  When the trace UID is unset, its value is NULL.

 - Add bt_trace_get_uid and bt_trace_set_uid, to be used with MIP >= 1.

 - Make bt_trace_get_uuid and bt_trace_set_uuid only available with
   MIP == 0.

 - Add BT_ASSERT_PRE_UID_NON_NULL, for use in bt_trace_set_uid.

 - Handle the trace UID in lib-logging.c:format_trace.

 - Free the UID in destroy_trace.

Philippe updated the documentation.

Change-Id: I3d84a9e6f04d632d187c8adfc4d625ce4c4357c0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9972
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>