cpp-common: add `bt2s::optional`, alias of `nonstd::optional`

cpp-common: add `bt2s::optional`, alias of `nonstd::optional`

In order to avoid having all sorts of external namespaces, make
`bt2s::optional` an alias of `nonstd::optional`, and make sure
everything in the project uses `bt2s::optional`.

Also add aliases for other STL types and functions of `<optional>`.

This will make it possible to easily change the implementation without
changing the code using it indirectly (only
`cpp-common/bt2s/optional.hpp` will change).

Move `cpp-common/optional.hpp` to
`cpp-common/vendor/optional-lite/optional.hpp` to make it clear it's an
external project.

Making everything under `cpp-common/vendor` an exception in
`tools/format-cpp.sh`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2a1ea52c484c85fbf8d68631d40369efd969818a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11391
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>