cpp-common/bt2: `bt2::internal::BorrowedObj` -> `bt2::BorrowedObj`

cpp-common/bt2: `bt2::internal::BorrowedObj` -> `bt2::BorrowedObj`

`BorrowedObj` is not completely internal: it has public methods which
any concrete borrowed object inherits.

Also, we don't need to hide anything from the end user in this file.

Therefore make this base class not internal.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib3a396a1aa7794608a4ec938f9b141c553e36c97
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11365
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>