cpp-common: Add hash() and isSame() methods to BorrowedObj

cpp-common: Add hash() and isSame() methods to BorrowedObj

This will allow us to use the C++ wrapper objects in
`std::unordered_map` by using these methods in custom hasher and isEqual
objects.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I4e018ece9ea4ca755f6612cd9adec408667b202d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7356
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10755
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>