lib: add internal BT_LIB_LOG*_APPEND_CAUSE() macros

lib: add internal BT_LIB_LOG*_APPEND_CAUSE() macros

The new BT_LIB_LOGE_APPEND_CAUSE() and BT_LIB_LOGF_APPEND_CAUSE()
functions are like BT_LIB_LOGE() and BT_LIB_LOGF(), but they also call
bt_current_thread_error_append_cause_from_unknown() with the formatted
message to append a cause to the current thread's error object.

I plan to use them in lieu of BT_LIB_LOGE() and BT_LIB_LOGF() where
those are already used within the library to make support error
reporting from the library with minimal changes.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6d7e3a5feac977f576c29c2451b5f0da8d42ae2b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1617
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>