Add `std::optional` replacement (`nonstd::optional`)

Add `std::optional` replacement (`nonstd::optional`)

This patch adds an `std::optional` replacement, `nonstd::optional`,
which comes from the Boost-licensed
<https://github.com/martinmoene/optional-lite> project.

Babeltrace 2 cannot use `std::optional` as it's part of C++17, whereas
the project only requires C++11.

`nonstd::optional` is meant to work like C++17's `std::optional`.

In any Babeltrace 2 C++ file, include it like this:

    #include "cpp-common/optional.hpp"

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Icda5af4bd5affd1237786bdb9cb2e64861643440
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4517