Add libbabeltrace2 C++ interface base

This patch adds three new C++ headers to `src/cpp-common/bt2/internal`:

`borrowed-obj.hpp`:
    Defines `bt2::internal::BorrowedObj` which wraps a libbabeltrace2
    object pointer without managing reference counting.

`shared-obj.hpp`:
    Defines `bt2::internal::SharedObj` which contains a
    `bt2::internal::BorrowedObj` instance and manages the underlying
    reference count of the libbabeltrace2 object.

Those classes are meant to be used by public classes in
`src/cpp-common/bt2`, hence their `internal` namespace.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I1433ef3c1ed9cca5677012ebe68f63545dbbe547
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4505