cpp-common/bt2: make `bt2::BorrowedObject::LibObj` public

cpp-common/bt2: make `bt2::BorrowedObject::LibObj` public

This patch changes the protected `bt2::BorrowedObject::_LibObjPtr`
to the public `bt2::BorrowedObject::LibObjPtr`.

There's already a public libObjPtr() method anyway which used to return
`_LibObjPtr`.

Also adding `LibObj` which is the object without the pointer part.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I23c841bdc9f58b4b3ecb9c7dee0403d49f048c1d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11437
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>