cpp-common: Expose BorrowedObj::libObjPtr() as public method

This will be useful for component class queries that may use bt2::Value
C++ wrappers but must return a `bt_value` at the moment.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Iea93caa6b47ce1afeb7977fee26dffdd3f97013e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7948
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10787
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>